Anna K Wåhlin
email: awahlin@gu.se

Nadine Steiger

Elin Darelius-Chiche

Karen M Assmann

Mirjam S Glessmer

Kyung Ho

Laura Ha

Céline Herraiz-Borreguero

Adrian Heuzé

Tae-Wan Jenkins

Kim

Jenkins A Kim

Ice front blocking of ocean heat transport to an Antarctic ice shelf

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introductory paragraph:

Mass loss from the Antarctic Ice Sheet to the ocean has increased in recent decades, largely because the thinning of its floating ice shelves has allowed the outflow of grounded ice to accelerate [START_REF] Shepherd | Trends and connections across the Antarctic cryosphere[END_REF][START_REF] Pritchard | Antarctic ice-sheet loss driven by basal melting of ice shelves[END_REF] . Enhanced basal melting of the ice shelves is thought to be the ultimate driver of change [START_REF] Pritchard | Antarctic ice-sheet loss driven by basal melting of ice shelves[END_REF][START_REF] Liu | Ocean-driven thinning enhances iceberg calving and retreat of Antarctic ice shelves[END_REF] , motivating a recent focus on the processes that control ocean heat transport onto and across the seabed of the Antarctic continental shelf towards the ice [START_REF] Thoma | Modelling Circumpolar Deep Water intrusions on the Amundsen Sea continental shelf , Antarctica[END_REF][START_REF] Wåhlin | Some Implications of Ekman Layer Dynamics for Cross-Shelf Exchange in the Amundsen Sea[END_REF][START_REF] Darelius | Observed vulnerability of Filchner-Ronne Ice Shelf to wind-driven inflow of warm deep water[END_REF] . However, the shoreward heat flux typically far exceeds that required to match observed melt rates [START_REF] Pritchard | Antarctic ice-sheet loss driven by basal melting of ice shelves[END_REF][START_REF] Palóczy | Oceanic Heat Delivery to the Antarctic Continental Shelf : Large-Scale , Low-Frequency Variability[END_REF][START_REF] Assmann | Warm Circumpolar Deep Water at the western Getz Ice Shelf Front, Antarctica[END_REF] , suggesting other critical controls. Here we show that the depth-independent (barotropic) component of the flow towards an ice shelf is blocked by the dramatic step shape of the ice front, and that only the depth-varying (baroclinic) component, typically much smaller, can enter the sub-ice cavity. Our results arise from direct observations of the Getz Ice Shelf system and laboratory experiments on a rotating platform. A similar blocking of the barotropic component may occur in other areas with comparable ice-bathymetry configurations, which may explain why changes in the density structure of the water column have been found to be a better indicator of basal melt rate variability than the heat transported onto the continental shelf [START_REF] Jenkins | West Antarctic Ice Sheet retreat in the Amundsen Sea driven by decadal oceanic variability[END_REF] . Representing the step topography of the ice front accurately in models is thus important for simulating the ocean heat fluxes and induced melt rates.

Main text:

The fate of the Antarctic Ice Sheet is the greatest remaining uncertainty when predicting future sea level [START_REF]Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF] . Estimates of its contribution to global sea-level rise range from none to a catastrophic > 5 cm/year [START_REF]Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF][START_REF] Imbieteam | Mass balance of the Antarctic ice sheet from 1992 to 2017[END_REF][START_REF] Deconto | Contribution of Antarctica to past and future sea-level rise[END_REF] (4 m by the year 2100). The ice sheet drains into the ocean where it terminates in floating ice shelves, overlying vast sub-ice cavities. These buttress the flow of the ice sheet, regulating the speed at which it flows into the ocean [START_REF] Scambos | Glacier acceleration and thinning after ice shelf collapse in the Larsen B embayment, Antarctica[END_REF] . Rapid thinning of ice shelves in coastal regions with warm ocean water on the continental shelf is accelerating the outflow from the ice sheet [START_REF] Shepherd | Trends and connections across the Antarctic cryosphere[END_REF][START_REF] Pritchard | Antarctic ice-sheet loss driven by basal melting of ice shelves[END_REF] . The perceived reason -although rarely observed directly [START_REF] Jenkins | Observations beneath Pine Island Glacier in West-Antarctica and implications for its retreat[END_REF] is that ocean currents deliver more warm water to the ice shelf cavities, causing increased basal melt. These currents originate in a reservoir of warm and salty water, known as Circumpolar Deep Water (CDW) [START_REF] Callahan | The structure and circulation of Deep Water in the Antarctic[END_REF] , residing at 300-1000 m depth in the Southern Ocean. Substantial amounts of dense CDW are carried onto the continental shelf by various mechanisms [START_REF] Thoma | Modelling Circumpolar Deep Water intrusions on the Amundsen Sea continental shelf , Antarctica[END_REF][START_REF] Wåhlin | Some Implications of Ekman Layer Dynamics for Cross-Shelf Exchange in the Amundsen Sea[END_REF][START_REF] Darelius | Observed vulnerability of Filchner-Ronne Ice Shelf to wind-driven inflow of warm deep water[END_REF][START_REF] Palóczy | Oceanic Heat Delivery to the Antarctic Continental Shelf : Large-Scale , Low-Frequency Variability[END_REF][START_REF] Thompson | The Antarctic Slope Current in a Changing Climate[END_REF] , but only a fraction of this is needed to explain observed basal melt rates [START_REF] Depoorter | Calving fluxes and basal melt rates of Antarctic ice shelves[END_REF] .

The CDW flows southward in deep troughs that crosscut the continental shelf [START_REF] Thoma | Modelling Circumpolar Deep Water intrusions on the Amundsen Sea continental shelf , Antarctica[END_REF][START_REF] Wåhlin | Inflow of Warm Circumpolar Deep Water in the Central Amundsen Shelf*[END_REF][START_REF] Herraiz-Borreguero | Circulation of modified Circumpolar Deep Water and basal melt beneath the Amery Ice Shelf, East Antarctica[END_REF][START_REF] Nakayama | Origin of Circumpolar Deep Water intruding onto the Amundsen and Bellingshausen Sea continental shelves[END_REF][START_REF] St-Laurent | On the Role of Coastal Troughs in the Circulation of Warm Circumpolar Deep Water on Antarctic Shelves[END_REF] . The currents are steered by the bathymetry and move with shallower water to the left of the flow direction [START_REF] Cushman-Roisin | Introduction to Geophyscial Fluid Dynamics: Physical and numerical aspects[END_REF][START_REF] Nof | The translation of isolated cold eddies on a sloping bottom[END_REF][START_REF] Wåhlin | Topographic advection of dense bottom water[END_REF] so southward transport occurs along the eastern, and northward on western, flanks of the troughs [START_REF] Herraiz-Borreguero | Circulation of modified Circumpolar Deep Water and basal melt beneath the Amery Ice Shelf, East Antarctica[END_REF][START_REF] Ha | Circulation and Modification of Warm Deep Water on the Central Amundsen Shelf[END_REF] . The flow is a combination of barotropic (vertically constant, winddriven [START_REF] Wåhlin | Variability of Warm Deep Water Inflow in a Submarine Trough on the Amundsen Sea Shelf[END_REF][START_REF] Kalén | Is the oceanic heat flux on the central Amundsen sea shelf caused by barotropic or baroclinic currents? Deep[END_REF] ) and baroclinic (vertically varying, density-driven) currents. Although the barotropic velocities often dominate [START_REF] Kalén | Is the oceanic heat flux on the central Amundsen sea shelf caused by barotropic or baroclinic currents? Deep[END_REF]28 , most of the heat is contained in the warm dense water below the thermocline where the baroclinic component typically enhances the flow.

In order to enter the ice shelf cavity the currents must pass the ice front -a wall of ice protruding from the surface to depths of 250 -500 m. This front imposes an abrupt change in the thickness of the water column, potentially disrupting the topographically steered flow towards it 29 . Logistical challenges generally prevent observations near the ice front, and estimates of oceanic heat transport towards the ice shelves are based on moorings placed at a 'safe' distance (at least a few km) away from the ice front.

To examine the effect of the ice front on the along-trough current, three moorings equipped with velocity profilers and loggers for temperature, salinity, and pressure were placed in a deep trough leading to Getz Ice Shelf (Fig. 1). Two of the moorings were positioned 14 km and 11 km away from the ice front at depths of 600 and 700 m respectively, while the third was placed 700-800 m from the front at 600 m depth. The ice front draft is 250-300 m 30 , and its position was constant during the two years of measurements (Fig. 1).

Feather-plots of the average velocity at various depths for the three moorings (Fig. 1, Methods, full time series in Extended Data Figs 123) show a persistent current up to 30 cm/s directed towards the ice shelf, parallel to the local bathymetry [START_REF] Assmann | Warm Circumpolar Deep Water at the western Getz Ice Shelf Front, Antarctica[END_REF] . The velocity at the near-front mooring was less than one third of those in the channel and deflected westward by up to 45 o .

Separating the currents into barotropic and baroclinic components (Fig. 2, Methods, Extended Data Figs 45) reveals that while GW1 and GW2 had significant barotropic along-slope flow (7.5 and 10 cm/s) with a baroclinic amplification in the warm bottom layer, the velocity at GW3 had a comparatively small barotropic component (0.1 cm/s) and was dominated by the baroclinic flow in the warm bottom layer. The direction of the baroclinic flow at GW3 is into the ice shelf cavity, i.e. parallel to the local topography and orthogonal to the ice front. It should be noted however that the bathymetry underneath the ice shelf has not yet been surveyed 31 . In the un-surveyed areas south of mooring GW3 the compilation used in Fig. 1 is based on gravity inversions associated with high uncertainty 31 . If there are underwater features such as submarine ridges and seamounts present underneath the ice shelf these might redirect the flow.

The strong correlation between the velocity at GW3 and the baroclinic velocities at GW1 and GW2 (Fig. 2 and Table 1, dark blue fields), indicates that the baroclinic current component at GW1 and GW2 is continuing to GW3. The barotropic component however has no significant correlation to the GW3 velocity, suggesting that it is diverted along the ice shelf front before it reaches GW3 (Fig. 1, Fig. 2). This is further evidenced by the high correlation between bottom temperature/density anomalies at GW2 and GW3 (both at the 600 m isobaths, Table 1, dark blue field). The barotropic component of the flow carries about 70% of the total heat transport (Extended Data Table 1, Extended Data Figure 6, Methods) at GW1 and GW2, similar to values on the central Amundsen Shelf [START_REF] Kalén | Is the oceanic heat flux on the central Amundsen sea shelf caused by barotropic or baroclinic currents? Deep[END_REF] , while at GW3 it carries only 3-10% (based on the more realistic methods (i) or (ii) for estimating barotropic velocity, see Methods). The heat transport is dominated by the mean flow rather than the fluctuations assessed in Table 1 (Extended Data Table 1).

The observed behavior of the velocity components at the ice front can be explained by geostrophic ocean dynamics [START_REF] Cushman-Roisin | Introduction to Geophyscial Fluid Dynamics: Physical and numerical aspects[END_REF]29 . Geostrophic currents are non-divergent and therefore flow parallel to lines of constant water column thickness, or, in the open ocean, lines of constant depth [START_REF] Cushman-Roisin | Introduction to Geophyscial Fluid Dynamics: Physical and numerical aspects[END_REF][START_REF] Wåhlin | Topographic advection of dense bottom water[END_REF] . This is the reason why the currents in the deep troughs are so strongly steered by the (comparatively gentle) topography. However, where a floating ice shelf with a considerable draft overlies the ocean, the water column thickness is no longer equal to the depth. Applied to the present setting this means that barotropic currents approaching the ice front along depth contours will be diverted due to the change in water column thickness (Methods) and may be blocked entirely without reaching the ice shelf cavity 29 . Baroclinic flow, on the other hand, can move along depth contours into the ice shelf cavity, provided the thermocline is deeper than the ice shelf draft.

In order to explore this phenomenon in a controlled environment, experiments were conducted in the 13-m diameter rotating Coriolis platform in Grenoble, France. A simplified bathymetry -a v-shaped trough -was placed in a 90-cm deep tank filled with fresh water (Fig. 3). A source was placed on the right flank (facing North) of the trough, pumping fresh water to set up a barotropic current, or saline (denser) water for a baroclinic bottom current. At the far end of the trough a plexiglass ice shelf with adjustable draft was placed. A detailed description of the experimental setup is presented in Methods.

The experimental results agree qualitatively with the geostrophic dynamics outlined above.

The current followed the trough flank towards the ice shelf, and away from it on the opposite side, in similarity with observations [START_REF] Herraiz-Borreguero | Circulation of modified Circumpolar Deep Water and basal melt beneath the Amery Ice Shelf, East Antarctica[END_REF][START_REF] Ha | Circulation and Modification of Warm Deep Water on the Central Amundsen Shelf[END_REF] (Fig. 4). Placing an ice shelf with near-zero draft on top of the trough (Fig. 4A) had no visible impact on the circulation. However, a sloping ice shelf with zero draft at the front and 30 cm at the back (Fig. 4B) caused the barotropic flow to change direction and follow lines of constant water thickness into the ice shelf cavity. A horizontal ice shelf with 30 cm draft (Fig. 4C) blocked the current from entering the cavity.

The baroclinic currents (Extended Data Fig. 9) continued mostly unaffected into the ice shelf cavity for all ice shelf drafts and shapes.

The observational and experimental results presented here enhance our understanding of how changes in oceanic heat transport on the continental shelf can impact basal melt. Barotropic flow is blocked, either partially or entirely, depending on the ice front geometry, from entering the cavity. Changes in the water temperature and/or baroclinic flow, on the other hand, will change the amount of heat that flows into the cavity. How much of it is ultimately used for basal melting depends on the cavity efficiency 32 . The results explain why changes in the thickness of the warm water layer seem to be a more reliable indicator of melt rate variability than e.g. ocean transports across the shelf break. Changes in the vertical structure of the water column is a better diagnostic of the critical baroclinic heat transport.

Since flows toward ice shelf cavities nearly always have a substantial barotropic component [START_REF] Assmann | Warm Circumpolar Deep Water at the western Getz Ice Shelf Front, Antarctica[END_REF][START_REF] Wåhlin | Variability of Warm Deep Water Inflow in a Submarine Trough on the Amundsen Sea Shelf[END_REF][START_REF] Kalén | Is the oceanic heat flux on the central Amundsen sea shelf caused by barotropic or baroclinic currents? Deep[END_REF]33 , the findings have broad implications for calculations of ocean heat transport to ice shelf cavities. For example, the measured heat transport along the Siple Trough is 2.27-2.8 TW (Extended Data Table 1) -sufficient to melt about 250-300 Gt/yr ice and twice the total basal melt, 136 Gt/yr, that the entire Getz ice shelf experiences [START_REF] Depoorter | Calving fluxes and basal melt rates of Antarctic ice shelves[END_REF] .

However, due to the abrupt front shape only one sixth (0.47 TW) of the heat that flows past GW1-2 enters the cavity. The results indicate that the floating ice shelves not only give backstress, mechanically slowing down the inland ice sheet [START_REF] Scambos | Glacier acceleration and thinning after ice shelf collapse in the Larsen B embayment, Antarctica[END_REF] , but that they also protect the vulnerable grounded ice by blocking a large portion of the warm ocean currents from reaching the cavity. The thickness and shape of the ice front may provide a critical and evolving control that needs to be incorporated accurately in models: Were an ice front to thin substantially, or to retreat back (or advance) to a region with larger underwater features steering the warm currents towards the cavity, then the heat flux to the ice sheet could change dramatically. Rare observations from inside the cavity [START_REF] Jenkins | Observations beneath Pine Island Glacier in West-Antarctica and implications for its retreat[END_REF]34 are needed to determine e.g. how much of the heat transport that eventually reaches the vulnerable grounding zones.

Gade, H. Melting of ice in sea water: A primitive model with application to the

Antarctic ice shelf and icebergs. J. Phys. Oceanogr. 9, 189-198 (1979).

Methods

Mooring data

Three moorings were deployed on 29 January 2016 and recovered on 18 January 2018 on the western flank of Siple Island (Fig. 1). Two of the moorings were deployed 11-14 km from the ice shelf at depths of 600 m (GW2, 73°47.6' S, 127°36.0'S) and 700 m (GW1, 73° 49. The along-slope directions were defined as true bearings of 135° for GW1, 110° for GW2, and 70° for GW3, based on the IBCSO 31 database (Fig. 1).

Ice shelf data

The position of the ice front shown in Figure 1 was manually digitized from Sentinel-1 Synthetic Aperture Radar images recorded in January of 2016, 2017 and 2018. Level-1

Ground Range Detected images, projected to ground range using the Earth ellipsoid model WGS84 with pixel size of 40x40 m. Getz ice shelf is characterized by surface structures parallel to the calving front 38 . This is the most common pattern observed among west Antarctic ice shelves and gives the type of calving front studied. The mean ice equivalent thickness of Getz ice shelf is 286 m 3 , comparable to the average of ice shelves in the Amundsen Sea (273 m). This indicates that Getz ice shelf is representative for the area.

Baroclinic and barotropic velocity components

According to thermal wind balance [START_REF] Cushman-Roisin | Introduction to Geophyscial Fluid Dynamics: Physical and numerical aspects[END_REF] the baroclinic velocity component is expected to be largest in the dense layer below the thermocline and small in the well-mixed water above it.

Since the present velocity data do not cover the upper water column (Extended Data Fig. 1) the barotropic (UBT) and baroclinic (UBC) velocity components have to be estimated based on the data at hand. Three different methods were employed and compared, (i) Assuming that the barotropic velocity component is given by the vertical average of the measured water column. While this method would give an accurate estimate in flows that have a comparatively thin baroclinic layer and/or a strong barotropic current, it will likely overestimate the barotropic current in the present data since only the bottom half of the water column is measured.

(ii) Assuming that the barotropic velocity component is given by the vertical average of the velocity from 150 m above the seabed to the upper end of the measured volume. This method will give an accurate estimate when the thermocline is closer than 150 m to the seabed but will otherwise overestimate the barotropic velocity component.

(iii) Assuming that the barotropic velocity component is given by the average velocity in the water above the thermocline. This method gives the most accurate result, but a disadvantage is that the thermocline was not always covered by the mooring data. By choosing the thermocline level to be at -0.5 o C, barotropic velocity estimates were obtained for nearly the complete record (Extended Data Fig. 1, lower panels).

Using any of the above methods, UBT and UBC can be calculated by 01 1 (Z Z )

1 0 BC BT Z BT Z U (z,t)= U(z,t) -U (t) U (t)= U(ξ,t)dξ - ∫ , (1)
where U(z,t) is the velocity measured at the moorings for various depths z and times t, ξ is the integration variable, and the integral limits Z0 and Z1 are given by one of the following 27 :

(i) Z0 = seabed and Z1 is the upper end of the measured water column.

(ii) Z0 = 150 m above the seabed and Z1 is the upper end of the measured water column (iii) Z0 is the -0.5 o C isotherm and Z1 is the upper end of the measured water column Extended Data Figure 4 shows time series of the three estimates (i) -(iii) of the barotropic velocities over the two years. Extended Data Figure 5 shows the average velocity (thick lines) together with the three alternative barotropic components (thin lines Extended Data Fig. 5A), the baroclinic component (Extended Data Fig. 5B) and the temperature (Extended Data Fig. 5C). In Figure 2 the barotropic velocity component was defined according to (ii) above, i.e.

red lines in Extended Data Figure 4 and dashed lines in Extended Data Figure 5A. Similar results were obtained using the other two definitions of Z0 and Z1, which is in accordance with [START_REF] Kalén | Is the oceanic heat flux on the central Amundsen sea shelf caused by barotropic or baroclinic currents? Deep[END_REF].

Heat transport calculations

Assuming that the width of the flow is bounded by the sloping topography (as suggested by the laboratory experiments), the heat transport H [J/s] toward the glacier can be estimated by ()

P REF D H W CUT T d η = ρ -ξ ∫ , (2)
where W [m] is the width of the sloping channel side, D is the bottom elevation, η is the top of the mooring, ρ [kg m -3 ] is density, CP [J K -1 kg -1 ] is the specific heat capacity, U [m s -1 ] is the (average) along-channel velocity, T [K] the temperature and TREF the temperature to which the water cools after interaction with glacial ice. Assuming that all the water cools to freezing temperature, ( 2) is given by ()

PF D H W C U T T dz η = ρ-∫ .
where TF [K] is the in situ freezing temperature (which decreases with pressure and salinity).

The heat flux induced by the barotropic respectively baroclinic velocity components is then given by BT BC HH H = + where ()

BC P BC F D H W C U T T dz η = ρ-∫ (3)
()

BT P BT F D H W C U T T dz η = ρ-∫ , (4)
and the barotropic (UBT) and baroclinic (UBC) velocity components are given by (1). In Extended Data Figure 6, time series of H, HBT and HBC were calculated using W = 10 km, CP = 3.968 kJ kg -1 K -1 , in situ freezing temperature 39 , in situ density 39 , and definition (ii) for the barotropic velocity (1). The temperature-and velocity data were re-gridded to a common grid using daily averages and linear interpolation in the vertical with 8 m cell size.

Extended Data Table 1 shows the temporal average of the heat flux calculated from ( 2) -( 4)

and each of the three methods (i) -(iii). As discussed, the barotropic velocity is likely overestimated with method (i) which gives smaller baroclinic heat flux components for all three moorings. The results of method (ii) and (iii) are quite consistent and shows that the baroclinic heat flux is about 30% at GW1 and GW2 while it is between 90% -97% at GW3, where the average barotropic velocity is nearly zero.

Heat transport errors

The instrument error in the ADCP is maximum 1.5 cm/s and the real error is significantly lower since an average over many pings was used. This error is of the same order of magnitude as the methodological uncertainty, exemplified by the three methods (Extended Data Fig. 5). In the conversion from velocity to heat transport there is an error involved in the assumption that the data at the mooring site is representative for the entire channel (equation ( 2)). In the absence of continuous, high resolution sampling across the width of the channel, which would enable an exact estimate of this error, an indication of the uncertainties involved can be obtained by the difference between the results of GW1 and GW2 (Table 1), i.e. about 0.5 TW or 18%. There is also an error caused by the fact that the upper part of the water column is not included in the heat flux calculations. Since the temperature above the measured volume is near freezing temperature (Extended Data Fig. 5), however, this error is relatively small.

Another source of error is the assumption that the flow is steady. By separating velocity and temperature into mean and fluctuating components the impact of temporal variability on the average heat transport can be estimated by

( ')( ' ) PF D H WC U U T T T d η =ρ + +- ξ ∫ , ( 5
)
where temporal mean is denoted by overbar and fluctuating part is denoted by hyphen. Since the temporal average of the fluctuating part is zero, (5) reduces to

( ( ) ' ') PF D H W C UT T UT d H H η = ρ - + ξ = + ∫  , ( 6
)
where H is the contribution from the average velocity and temperature, and H  is the contribution from the temporal variability about the mean. Extended Data Table 1 shows the two contributions -the heat flux in all three moorings is caused primarily by the mean and the contribution from the fluctuations is between 6% and 20%.

Theory

In geostrophic flow [START_REF] Nakayama | Origin of Circumpolar Deep Water intruding onto the Amundsen and Bellingshausen Sea continental shelves[END_REF] the momentum equations are dominated by the Coriolis-and the pressure gradient terms, i.e.

1 p v fx ∂ = ∂ ρ (7) 1 p u fy ∂ = - ∂ ρ , (8)
where (,) uv are the velocity components in the (, ) xy directions, f (s -1 ) is the Coriolis parameter and p is the hydrostatic pressure. Assuming that the Coriolis parameter is constant and using the Boussinesq approximation [START_REF] Cushman-Roisin | Introduction to Geophyscial Fluid Dynamics: Physical and numerical aspects[END_REF] , it follows from ( 7) -( 8) that geostrophic velocity is non-divergent, i.e.

0 uv xy ∂∂ += ∂∂ . ( 9
)
For the simplified case of one active layer, i.e. a well-mixed layer extending from the bottom to either the surface or to the interface separating an active dense layer from an inactive lighter water mass above it, vertical integration of the continuity equation gives [START_REF] Nakayama | Origin of Circumpolar Deep Water intruding onto the Amundsen and Bellingshausen Sea continental shelves[END_REF][START_REF] St-Laurent | On the Role of Coastal Troughs in the Circulation of Warm Circumpolar Deep Water on Antarctic Shelves[END_REF][START_REF] Cushman-Roisin | Introduction to Geophyscial Fluid Dynamics: Physical and numerical aspects[END_REF] (using (9) and the fact that the velocities are vertically homogeneous)

0 DD uvu v t xy xy ηηη ∂∂∂∂∂ ++--= ∂∂∂ ∂∂ , (10)
where η is the upper surface (either the water surface or the dense interface) and D is the bottom elevation. Equation (10) can also be expressed in terms of the layer thickness

Steady solutions to (11) have streamlines parallel to lines of constant water column thickness (H), irrespective of the bottom elevation D(x,y) and the pressure (as long as the flow is geostrophic). Equation ( 11) might appear trivial but the combination of geostrophy and solid upper and lower boundaries has important consequences for the currents entering ice shelf cavities in Antarctica. When an ice shelf is protruding from above, the along-trough flow experienced outside the cavity will be deflected to flow along the ice front instead. Barotropic flow towards Antarctica's ice shelves is thus expected to be blocked from reaching the inner parts of the ice shelf cavities (as seen in Fig. 1). Baroclinic flow, on the other hand, is expected to follow the depth contours into the inner ice shelf cavity.

Laboratory experiments

The laboratory experiments were conducted on the 13-m-diameter rotating platform at Laboratoire des Écoulements Géophysiques et Industriels (LEGI) in Grenoble, France.

A v-shaped channel of size 5 m × 1 m × 0.5 m and a 2% slope (Extended Data Fig. 7) was built at the center of the turntable (red dot Extended Data Fig. 7). Focusing on the dynamics of the flow and ignoring thermodynamic changes such as melting and freezing of ice, a cuboid Plexiglas ice shelf with adjustable elevation and tilt was placed at the lower (closed)

end of the channel. The tank was filled with 90 cm of fresh water and rotated clockwise (Southern Hemisphere) with a rotation period of 30 s, giving a Coriolis parameter f = 0.42 s -1 .

A source, placed in the center of the left-hand flank of the channel (looking towards the ice shelf) and resting on the topography, pumped water at 60 l/min into the channel. The source was 0.15 m high, 0.25 m wide, 0.25 m long and sloped at the bottom to fit the topography (Extended Data Fig. 7). The outflow area was 0.47 m² and had a honeycomb of small tubes to produce a homogeneous laminar flow. For the barotropic experiments the source water was fresh like the ambient water and for the baroclinic experiments it was saline and 2 kg m -3 denser than the ambient water. A drainage and skimmer kept the water level constant.

Neutrally buoyant particles (60 μm Dantec Dynamics particles) in the source water were illuminated by a horizontal laser plane (Extended Data Fig. 8) in order to visualize the flow.

Two cameras with pixel resolution 2560 × 2160 pixels were mounted above the channel. The (https://www.mn.uio.no/math/english/people/aca/jks/matpiv), and found to agree with UVMAT. Using the pixel per image value, i.e. 0.6 mm/0.5 s for barotropic (every 5 images were used) and 0.6 mm/0.1 s for baroclinic experiments, the velocity error was 1.2 mm/ for the barotropic and 6 mm/s for the baroclinic experiments. The obtained 25 (or 19 for baroclinic experiments) velocity fields for each level were then averaged, which lowered the error further. Figure 4 shows the average of 4-5 cycles at one level, starting at the time when the leading edge reached the ice front, together with the temporal standard deviation of the velocity for that level (cyan arrows) and the error (magenta bars). Outliers (defined as velocities for which the standard deviation exceeds 10 times the average standard deviation)

were identified and filtered out. The vertical sections (Fig. 4d-f) were created from the parts of the horizontal slices that occupied +/-2 cm around the green and magenta lines in Fig. 4.

In addition to the top-view cameras, a side-view camera was mounted outside a glass wall at the side of the tank and GoPro cameras were lowered into the water to get side-view images (Fig. 3 and Extended Data Fig. 8). In the side view images, fluorescent dye (rhodamin) was used for visualization.

The topography was built to mimic a submarine trough topography with depth variations of same magnitude as the ice shelf draft, in similarity with the observations. Geostrophic balance was ensured by choosing flow-and rotation rates so that both the Ekman number Ek (i.e. the frictional force compared to the Coriolis force [START_REF] Nakayama | Origin of Circumpolar Deep Water intruding onto the Amundsen and Bellingshausen Sea continental shelves[END_REF] ) and the Rossby number [START_REF] Nakayama | Origin of Circumpolar Deep Water intruding onto the Amundsen and Bellingshausen Sea continental shelves[END_REF] (i.e. the inertial forces compared to the Coriolis force) were smaller than one. The values of the various scales and the non-dimensional numbers are shown in Extended Data Table 1. While the Ekman number was clearly negligible (0.002-0.004), the Rossby number was 0.14-0.2 meaning that ageostrophic effects may amend the process, particularly in regions where the velocity might be larger.

Before each experiment the platform was spun up for 2-3 hours to reach solid body rotation, which was determined by observing the movement of particles. Each experiment was started by opening the source. After about 5 -10 minutes (faster for baroclinic flow) a current moving towards the ice shelf developed over the sloping part of the topography (Extended Data Fig. 8). Behind the leading edge of the current a semi-steady flow with regions of slower and faster flow moving in the direction of the ice shelf developed (Extended Data Fig. 8d).

After interaction with the ice-shelf (15-30 min after experiment start) a counter-current on the opposite side developed, after which the experiment ended.

The baroclinic flow developed faster, was more steady, and was not influenced by the presence of the ice shelf. Instead of returning on the opposite side, the baroclinic flow slowly filled the ice shelf cavity with dense water (Extended Data Fig. 8). Observational parameters for velocity and density difference were obtained from the GW1 and GW2 mooirng data, while the bathymetric parameters were obtained from [31]. The viscosity scale is a bulk eddy viscosity [START_REF] Cushman-Roisin | Introduction to Geophyscial Fluid Dynamics: Physical and numerical aspects[END_REF] .

  29. Grosfeld, K., Gerdes, R. & Determann, J. Thermohaline circulation and interaction between ice shelf cavities and the adjacent open ocean. J. Geophys. Res. Ocean. 102, 15595-15610 (1997). 30. Jacobs, S. et al. Getz Ice Shelf Melting Response to Changes in Ocean Forcing. J. Geophys. Res. -Ocean. 118, 4152-4168 (2013). 31. Arndt, J. E. et al. The International Bathymetric Chart of the Southern Ocean (IBCSO) Version 1.0 -A new bathymetric compilation covering circum-Antarctic waters. Geophys. Res. Lett. 40, 311-3117 (2013). 32. Little, C. M., Gnanadesikan, A. & Oppenheimer, M. How ice shelf morphology controls basal melting. J. Geophys. Res. 114, C12007 (2009). 33. Miles, T. et al. Glider observations of the Dotson Ice Shelf outflow. Deep Sea Res. Part II Top. Stud. Oceanogr. 123, 16-29 (2015). 34. Nicholls, K. W. et al. Measurements beneath an Antarctic ice shelf using an autonomous underwater vehicle. 33, 2-5 (2006).

Figure 1 .

 1 Figure 1. Blocking of topographically steered current at the Getz Ice Shelf front. (a) Mooring locations and time averaged velocities from three moorings (GW1-3) are shown as feather plots on top of the local bathymetry 31 . Velocities are color coded with conservative temperature θ and depth-averaged in 50 m bins starting at the bottom. The lowermost (red, warmest) and uppermost (blue, coldest) bin depths are quoted near the corresponding arrow. Also shown is the location of the ice front in January 2016, 2017 and 2018 (blue lines, Methods). Lower panels show conservative temperature θ versus absolute salinity SA for (b) GW1 (c) GW2 (d) GW3 in green hues, gray dots are the data from all moorings. Red squares indicate Circumpolar Deep Water temperature-and salinity range 15 , blue thick line is the mixing line between CDW and glacier meltwater 35 , lower black thin line is the freezing point (Tf). The lack of data points near salinity 34.5 g kg -1 in GW2 is due to the fact that GW2 only had two salinity sensors (Extended Data Figure 2), of which one was faulty for a period of time (see Methods). Mooring temperature-and velocity time series are shown in Extended Data Figs. 1 -3.

Figure 2 :

 2 Figure 2: Baroclinic velocity component at GW2 is similar to total velocity at GW3. Three-day average along-slope velocity (color bar, m/s), with isotherms (black contours, every 0.5 degrees, thick black line shows the 0 degree isotherm) (a) Total alongslope velocity at GW2 (b) Baroclinic velocity component (Methods) at GW2 (c) Total alongslope velocity at GW3. Note that the topmost sensor on GW2 was at 357 m depth while at GW3 it was at 288 m depth (Extended Data Figure 2, Extended Data Figure 3).

Figure 3 .

 3 Figure 3. Experimental set-up and difference between barotropic and baroclinic flow.(a) Sketch of the experiment. Side view sketches of the ice shelf (light gray), bottom (dark gray) and water (blue) with ice shelf draft 0 cm (b) 30 cm (c) and tilted (d). Photographs are from from underneath the ice shelf, facing out, for (e) barotropic flow and (f) baroclinic flow.

Figure 4 :

 4 Figure 4: Blocking of depth-independent currents in laboratory. Horizontal velocities from the laboratory experiments are presented for the barotropic flow with the three different ice shelf configurations (Fig. 3b-d). Colors indicate velocity in the y-direction, arrows indicate velocity vectors. (a)-(c) show velocities at the horizontal plane in the center of the current (black lines in (d)-(i)), (d)-(f) show velocities at vertical sections underneath the ice shelf

  8' S, 127° 47.6'S). The third mooring was located 700-750 m from the ice shelf at a depth of 600 m (GW3, 73° 50.0' S, 127°16.6'S), within a Rossby radius (2 km) of the ice front. The moorings were equipped with sensors for temperature, conductivity and pressure from Seabird Electronics (SBE37, SBE39 and SBE56) and Acoustic Doppler Current Profilers (ADCP, Teledyne RD Instruments, 75 and 150 kHz kHz Sentinel). The initial accuracy of the temperature data were 0.002 o C and the resolution was 0.0001 o C. The ADCP data were quality controlled using standard criteria for filtering out bad data and outliers 36 based on quality controls on individual beams and bins recorded by the instrument each ping (percent good returns below 50%, average echo intensity below 40 (counts) and roll and pitch of instrument exceeding 20 o filtered out). The raw data (saved at 15 minute temporal resolution) had standard error 1 -1.5 cm/s and were averaged to hourly means.Hydrographic measurements extended from the bottom to 357 and 305 m below the surface for GW2 and GW1, respectively, with downward looking ADCPs just above the top sensor, and to 288 m below the surface at GW3, with an upward looking ADCP just below the bottom sensor (Fig.1). Extended Data Figures1 -3show the North-and Eastward velocities recorded at the three locations, together with temperature. Conservative temperature and absolute salinity in Fig.1were calculated following TEOS-10 37

  footprint of both cameras (Exended Data Fig.7) gave a resolution of 0.6 mm/pixel. The laser shifted through depth levels starting near the bottom of the channel. For the barotropic experiments 12 different depth levels were used with a vertical distance of dz = 6.2 cm. In order to resolve better the faster-moving dense current and focus on the lower part of the channel, 7 different depth levels with dz = 5.8 cm were used in the baroclinic experiments. At each level, 30 (barotropic experiments) or 20 (baroclinic experiments) consecutive images were taken by both cameras with 0.1 s interval giving a total of 60 s for a complete cycle through all depth levels. The obtained images were used for Particle Image Velocimetry (PIV) calculations with the UVMAT software developed at LEGI (for details see http://servforge.legi.grenoble-inp.fr/projects/soft-uvmat). Independent results were also

Extended Data Figure 1 :Extended Data Figure 2 :Extended Data Figure 3 :Extended Data Figure 5 :Extended Data Figure 6 :Extended Data Figure 7 .Extended Data Figure 8 :Extended Data Figure 9 .Extended Data Table 1 :

 123567891 40. Darelius, E., Fer. I., Assmann, K. M. & Kim, T. W. Physical oceanography from Mooring UiB1 and UiB4 in the Amundsen Sea. (2018). doi:10.21335/NMDC-Two year time series of velocity and temperature from GW1 mooring. Time series of (a) eastward velocity, (b) northward velocity and (c) temperature for the GW1 mooring. Black lines in (c) indicate positions of Microcats (thick lines) and SBE56 (thin lines). Two year time series of velocity and temperature from GW2 mooring. Time series of (a) eastward velocity, (b) northward velocity and (c) temperature for the GW2 mooring. Black lines in (c) indicate positions of Microcats (thick lines) and SBE56 (thin lines). Two year time series of velocity and temperature from GW3 mooring. Time series of (a) eastward velocity, (b) northward velocity and (c) temperature for the GW3 mooring. Black lines in (c) indicate positions of Microcats (thick lines) and SBE56 (thin lines). Extended Data Figure 4: Comparison of methods for calculating barotropic component. Along-slope barotropic current component based on option (i): vertical average, option (ii): vertical average of the water more than 150 m above seabed, and option (iii): vertical average of water above the -0.5 o isotherm according to legend. (a) Mooring GW1, 3-day averaged (b) Mooring GW2, 3-day averaged (c) Mooring GW3, 3-day averaged. The barotropic velocity is larger for GW1 and GW2 than GW3, the baroclinic velocity and the temperature increase towards the bottom. (a) Thick lines show average along-slope velocities as a function of distance above bottom, with colors indicating mooring (legend). Thin vertical lines show the barotropic components estimated according to method (i) (dotted lines), method (ii) (dashed lines), and method (iii) (solid lines). (b) Baroclinic velocity components as a function of distance above bottom. (c) Average temperature as a function of distance above bottom. The barotropic heat flux component is larger than the baroclinic for GW1 and GW2. Time series of total heat flux and the barotropic and baroclinic components using expression (2) and definition (ii) of barotropic velocity. (a) Mooring GW1 (b) Mooring GW2 (c) Mooring GW3. Experiment set-up and dimensions. (a) Top view drawing of vshaped channel (blue), ice shelf (white), camera views (PCO1, green, PCO2, orange) and the source (to scale). (b) Side view drawing looking into the ice shelf facing South (c) Side view drawing looking East (d)-(f) Top views of topography (gray scale, color bar) and water column thickness (colored lines, labels) for (d) Ice shelf draft 0 cm (e) Ice shelf draft 30 cm, tilted (f) ice shelf draft 30 cm, horizontal. Photographs from the experiments. (a) Top view showing the experimental set-up with the horizontal and vertical laser sheets. (b) Technicians and students preparing for an experiment (c) Time series showing the ice shelf cavity filling up with dense water for the baroclinic experiments (d) Top view photograph showing a barotropic current moving towards the ice shelf. No blocking of depth-varying currents in laboratory. Horizontal velocities from the laboratory experiments are presented for the baroclinic flow with the three different ice shelf configurations (Fig. 3b-d). Colors indicate velocity in the y-direction, arrows indicate velocity vectors. (a)-(c) show velocities at the horizontal plane in the center of the current (black lines in (d)-(i)), (d)-(f) show velocities at vertical sections underneath the ice shelf (green lines in (a)-(c)) and (g)-(i) in front of it (magenta lines in (a)-(c)). Dashed and shadowed rectangles indicate the ice shelf, grey shading indicates topography and grey lines are bathymetric lines that the current is expected to follow. White areas are not measured/ missing data. The cyan arrow beneath the scale arrow in (a) -(c) indicate the temporal standard deviation of the velocity and magenta bar indicates the error (Methods). Part of heat flux caused by the barotropic current component is large compared to that caused by the baroclinic component. Average heat flux (H) and its barotropic (eq. (3)) and baroclinic (eq. (4)) components using different definitions of barotropic velocity (i) Vertical average over the entire measured water column (ii) Vertical average over the measured water column more than 150 m above the bottom (iii) Vertical average over the measured water column above the -0.5 o isotherm (see Methods). Also shown is the part of the heat flux induced by the average velocity and temperature ( H ) and their fluctuating components ( H  ) according to equation(6).Extended Data Table2: Non-dimensional scales are similar in laboratory experiment and observations. Scale values for velocity (U), density difference ( ∆ρ ), Coriolis parameter (f), depth (H), width (L), molecular (in laboratory) or turbulent (in field) viscosity (v) and the derived Ekman depth ( E δ ), Ekman number (Ek), Rossby radius (LR) and Rossby number (Ro).

Acknowledgments:

The experiments were supported by the European Community's H2020 through the grant to HYDRALAB-PLUS, Contract no. 654110. We gratefully acknowledge grant support for the authors as follows: ED and NS, Norwegian Research Council through grant no. 267660 (TOBACO) and 231549 (WARM); AJ from the UK Natural Environment Research Council through grant no. NE/L013770/1 (FISS), AW from the Swedish Research Council and the Swedish Foundation for Strategic Research through the Swedish Maritime Robotics Center (SMaRC), TWK from the Korea Polar Research Institute grant KOPRI PE19060. Thanks to I.

Fer for lending instrumentation to GW1 and GW2.

Data and code availability:

The mooring data analysed during the current study (raw data for Figure 1-2 andextended data Figures 123456) are available at the Norwegian Marine Data Centre (https://doi.org/10.21335/NMDC-1721053841 40 , GW1-2) and at SOOS data base at NODC

Author contributions:

AKW proposed the research. AKW, NS, ED and KA wrote the first draft. JS assisted with analyses and repository of laboratory data. All authors contributed to the laboratory experiments, to data processing, and/or to the field work. AKW, NS, SV, AKM prepared the figures. All authors read and commented on the text.

Author information:

Corresponding author: Dr. A.K. Wåhlin, email: awahlin@gu.se No authors have any competing interests.

Author affiliations:

(https://doi.org/10.25921/n07g-f935 and https://doi.org/10.25921/6pwp-1791, GW3) Raw data obtained from the PIV calculations (raw data for Figure 4 and Extended Data Figure 9) are available at Zenodo (https://zenodo.org/record/3543624).

The PIV calculations were conducted with the matlab software UVMAT developed at LEGI available at http://servforge.legi.grenoble-inp.fr/projects/soft-uvmat. Independent results were also obtained with the MatPIV package available at https://www.mn.uio.no/math/english/people/aca/jks/matpiv.

Extended data legends

Extended Data Figure 1 3)) and baroclinic (eq. ( 4)) components using different definitions of barotropic velocity (i) Vertical average over the entire measured water column (ii) Vertical average over the measured water column more than 150 m above the bottom (iii) Vertical average over the measured water column above the -0.5 o isotherm (see Methods). Also shown is the part of the heat flux induced by the average velocity and temperature ( H ) and their fluctuating components ( H  ) according to equation (6).

Extended Data


Canliang Zhou

Linfeng Sun

Fengquan Zhang

Chenjie Gu
email: guchenjie@nbu.edu.cn

Shuwen Zeng
email: shuwen.zeng@unilim.fr.

Tao ||

Jiang

Xiang Shen

Diing Shenp

Jun Zhou
email: zhoujun@nbu.edu.cn

Electrical Tuning of the SERS Enhancement by Precise Defect Density Control

Keywords: surface enhanced Raman scattering, transition metal oxide, chemical enhancement, charge transfer, electrical programing, defect density tuning

Surface-enhanced Raman scattering (SERS) has been widely established as a powerful analytical technique in molecular fingerprint recognition. Although conventional noble metal-based SERS substrates show admirable enhancement of the Raman signals, challenges on reproducibility, bio-compatibility and costs limit their implementations as the preferred analysis platforms. Recently, researches on SERS substrates have found that some innovatively prepared metal oxides/chalcogenides could produce noble metal comparable SERS enhancement, which profoundly expanded the material selection. Nevertheless, to tune the SERS enhancement of these materials, careful experimental designs and sophisticated processes were needed. Here, it is demonstrated an electrically tunable SERS substrate based on tungsten oxides (WO 3-x ). Electric field is used to introduce the defects in the oxide on an individual substrate, readily invoking the SERS detection capability, and further tuning the enhancement factor is achieved through electrical programming of the oxide leakage level. Additionally, by virtue of in-situ tuning the defect density and enhancement factor, the substrate can adapt to different molecular concentrations, potentially improving the detection range. These results not only help build a better understanding of the chemical mechanism, they also open an avenue for engaging nonnoble metal materials as multi-functional SERS substrates.

The surface-enhanced Raman scattering (SERS) technique has been intensively studied since it was first observed on a roughened silver electrode coated with pyridine in 1973. [START_REF] Fleischmann | Raman Spectra of Pyridine Adsorbed at a Silver Electrode[END_REF] It is known for ultra-high sensitivity and specificity when it is used to detect the molecules adsorbed on the SERS substrate. For this reason, SERS detection has been extensively utilized as a powerful tool for molecular fingerprint tracing in various biological and chemical applications. [START_REF] Yu | Sensitive and Simple Determination of Zwitterionic Morphine in Human Urine Based on Liquid-Liquid Micro-Extraction Coupled with Surface-Enhanced Raman Spectroscopy[END_REF][START_REF] Xu | Plasmon-Triggered Hot-Spot Excitation on SERS Substrates for Bacterial Inactivation and in Situ Monitoring[END_REF][START_REF] Zhou | Simultaneous Surface-Enhanced Raman Spectroscopy Detection of Multiplexed Micro RNA Biomarkers[END_REF][START_REF] Hu | Surface-Enhanced Raman Scattering Active Gold Nanoparticles with Enzyme-Mimicking Activities for Measuring Glucose and Lactate in Living Tissues[END_REF][START_REF] Lane | SERS Nanoparticles in Medicine: From Label-Free Detection to Spectroscopic Tagging[END_REF][START_REF] Feng | Development of Molecularly Imprinted Polymers-Surface-Enhanced Raman Spectroscopy/Colorimetric Dual Sensor for Determination of Chlorpyrifos in Apple Juice[END_REF] As the SERS performance is usually determined by material selection, surface morphology of the substrate and the adsorbed analytes, developing novel substrate with attractive enhancement factor (EF), excellent uniformity and reproducibility, additional bio-compatibility and reusability is keenly demanded. [START_REF] Ding | Nanostructure-Based Plasmon-Enhanced Raman Spectroscopy for Surface Analysis of Materials[END_REF][START_REF] Schlücker | Surface-Enhanced Raman Spectroscopy: Concepts and Chemical Applications[END_REF] Conventionally, noble metals with optimized surface nanostructures are implemented as the SERS substrates and normally up to 7~8 orders of molecules' Raman scattering signal enhancement can be obtained. [START_REF] Jiang | Construction of Long Narrow Gaps in Ag Nanoplates[END_REF][START_REF] Niu | Highly Symmetric Gold Nanostars: Crystallographic Control and Surface-Enhanced Raman Scattering Property[END_REF][START_REF] Liu | Porous Au-Ag Nanospheres with High-Density and Highly Accessible Hotspots for SERS Analysis[END_REF][START_REF] Maiorano | Monodispersed and Size-Controlled Multibranched Gold Nanoparticles with Nanoscale Tuning of Surface Morphology[END_REF] The mechanism behind the enhancement observed on noble metal substrates is attributed to the boosted surface electromagnetic field confinement by the surface plasmon, and thus it is termed as the electromagnetic mechanism (EM) for the Raman signal enhancement. [START_REF] Kneipp | Single Molecule Detection Using Surface-Enhanced Raman Scattering (SERS)[END_REF][START_REF] Xu | Spectroscopy of Single Hemoglobin Molecules by Surface Enhanced Raman Scattering[END_REF] However, to make full use of the EM, precise control of the metal nanostructures to ensure the strong near-surface electromagnetic field still poses as a significant challenge, especially known for sophisticated experimental design and high costs. In addition, noble metals occasionally show poor stability and biocompatibility when the SERS detection is carried in a harsh environment or biological system, which further limits their applications as the prior-selection analysis platforms. [START_REF] Millo | Characterization of Hybrid Bilayer Membranes on Silver Electrodes as Biocompatible SERS Substrates to Study Membrane-Protein Interactions[END_REF][START_REF] Runowski | Preparation of Biocompatible, Luminescent-Plasmonic Core/Shell Nanomaterials Based on Lanthanide and Gold Nanoparticles Exhibiting SERS Effects[END_REF][START_REF] Wang | Evaluating Cellular Uptake of Gold Nanoparticles in HL-7702 and HepG2 Cells for Plasmonic Photothermal Therapy[END_REF][START_REF] Potara | Chitosan-coated Triangular Silver Nanoparticles as a Novel Class of Biocompatible, Highly Sensitive Plasmonic Platforms for Intracellular SERS Sensing and Imaging[END_REF] Recently, studies on transition metal oxides (TiO 2 , Cu 2 O, MoO 3 , ZnO and WO 3 etc.), transition metal chalcogenides (Cu 2 S, Cu 2 Se, MoS 2 and WSe 2 etc.) and two-dimensional materials (graphene, graphene oxide etc.) have demonstrated that non-metal surface could also provide noble-metal comparable SERS enhancement. [START_REF] Sun | Plasma Modified MoS 2 Nanoflakes for Surface Enhanced Raman Scattering[END_REF][START_REF] Jiang | A Sensitive SERS Substrate Based on Au/TiO 2 /Au Nanosheets[END_REF][START_REF] Yang | Investigation on SERS of Different Phase Structure TiO 2 Nanoparticles[END_REF][START_REF] Jiang | Double Metal Co-Doping of TiO2 Nanoparticles for Improvement of their SERS Activity and Ultrasensitive Detection of Enrofloxacin: Regulation Strategy of Energy Levels[END_REF][START_REF] Yu | Graphene/TiO 2 Hybrid Layer for Simultaneous Detection and Degradation by a One-Step Transfer and Integration Method[END_REF][START_REF] Liu | Enhanced Raman Scattering of CuPc Films on Imperfect WSe 2 Monolayer Correlated to Exciton and Charge-Transfer Resonances[END_REF][START_REF] Liu | Improved Surface-Enhanced Raman Spectroscopy Sensitivity on Metallic Tungsten Oxide by the Synergistic Effect of Surface Plasmon Resonance Coupling and Charge Transfer[END_REF][START_REF] Ou | Tuning Defects in Oxides at Room Temperature by Lithium Reduction[END_REF] In these experiments, the Raman signal enhancement was mainly ascribed to an increase of charge transfer (CT) between the substrate material and molecules, consequently enhancing the molecule polarizability. Thus, the mechanism is named as the chemical mechanism (CM) for the Raman signal enhancement. [START_REF] Otto | The 'Chemical' (Electronic) Contribution to Surface-Enhanced Raman Scattering[END_REF] In the meanwhile, a series of elegant papers also have reported that materials like TiO 2 , MoO 3 , ZnO, WO 3 , graphene and MoS 2 showed excellent biocompatibility, and some of them were highly resistant to degradation in harsh environment, therefore, the potential use of these materials would greatly expand the application of SERS detection in many fields, e.g. in-vivo health monitoring or chemical reaction tracing. [START_REF] Yin | High-Throughput Synthesis of Single-Layer MoS 2 Nanosheets as a Near-Infrared Photothermal-Triggered Drug Delivery for Effective Cancer Therapy[END_REF][START_REF] Mehrali | Synthesis, Mechanical Properties, and in Vitro Biocompatibility with Osteoblasts of Calcium Silicate-Reduced Graphene Oxide Composites[END_REF][START_REF] Wen | Ultrasmall Biocompatible WO 3-x Nanodots for Multi-Modality Imaging and Combined Therapy of Cancers[END_REF][START_REF] Hurst | Utilizing Chemical Raman Enhancement: A Route for Metal Oxide Support-Based Biodetection[END_REF][START_REF] Ji | Semiconductor Materials in Analytical Applications of Surface-Enhanced Raman Scattering[END_REF][START_REF] Zhou | Dissolving Behavior and Stability of ZnO Wires in Biofluids: A Study on Biodegradability and Biocompatibility of ZnO Nanostructures[END_REF] However, essential improvements of the non-metal SERS substrates, such as a simple preparing procedure and achievement of high EF, are still demanded.

To-date, established strategies for promoting the CM in SERS lie on the alignment of energy levels between the substrate material and molecules to facilitate the CT. Electrochemical SERS, as a comprehensively discussed technique, has been implemented to manipulate the surface states and promote the CT for SERS enhancement, however this method requires constant voltage bias and aqueous electrolyte. [START_REF] Avila | How the Electrode Potential Controls the Selection Rules of the Charge Transfer Mechanism of SERS[END_REF][START_REF] Wu | Electrochemical Surface-Enhanced Raman Spectroscopy of Nanostructures[END_REF] Recently, defect engineering on the solid-state materials has been extensively used to improve the SERS performance of materials. Compared to the electrochemical method, the defect states in these solid-state electrolytes are nonvolatile, which means that they are able to support the CT even after the removing of the bias. To introduce the defect states in the material, previous reported results have indicated that plasma treatment, ion doping or gas annealing, etc. on materials like TiO 2 , WO 3 and ZnO could be effective approaches. [START_REF] Yang | Investigation on SERS of Different Phase Structure TiO 2 Nanoparticles[END_REF][START_REF] Cong | Noble Metal-Comparable SERS Enhancement from Semiconducting Metal Oxides by Making Oxygen Vacancies[END_REF][START_REF] Sinha | Recyclable SERS Substrates Based on Au-Coated ZnO Nanorods[END_REF][START_REF] Cao | Atomic-Layer-Deposition Assisted Formation of Wafer-Scale Double-Layer Metal Nanoparticles with Tunable Nanogap for Surface-Enhanced Raman Scattering[END_REF] In the meanwhile, high-energy ultraviolet (UV) light was also implemented to create the defects in the substrate oxide. A UV-exposed Au-nanoparticle embedded oxide substrate showed synergetic enhancement effect; the Raman intensity was raised to a much higher level than the case without UV exposure. [START_REF] Ben-Jaber | Photo-Induced Enhanced Raman Spectroscopy for Universal Ultra-Trace Detection of Explosives, Pollutants and Biomolecules[END_REF] Furthermore, optimization of chemical synthesis conditions was another extensively adopted approach for adjusting the surface states and achieving significant Raman signal improvement. [START_REF] Yu | Graphene/TiO 2 Hybrid Layer for Simultaneous Detection and Degradation by a One-Step Transfer and Integration Method[END_REF][START_REF] Yin | Significantly Increased Raman Enhancement on MoX 2 (X=S, Se) Monolayers upon Phase Transition[END_REF][START_REF] Wu | Metal Oxide Semiconductor SERS-Active Substrates by Defect Engineering[END_REF] All in all, these experimental practices purposely introduce defects into the material and the SERS enhancement subsequently benefits from the trap-assisted CT between the substrate material and molecules. Nevertheless, they involve a time-consuming process and some of them even require accuracy control of processing conditions. Besides, the defect density as well as the defect level positions in the substrate material is relatively difficult to control through these methods.

Herein, we report the study of the electrically tunable Raman enhancement phenomenon based on the design and fabrication of tungsten oxides (WO 3-x ) associated substrate. By electrical programing the defect density in the WO 3-x through the oxide leakage current control, the SERS detection capability of the novel substrate can be invoked instantaneously and the enhancement factor can be further precisely modulated. Moreover, the advantage of in-situ electrical defect density control allows the EF of the SERS substrate that can be tuned and adapted to different molecular concentrations. This gives the SERS substrates a self-adapt capability with potentially improved detection range. The above results not only help affirm the CM, they also pave the way towards the implementation of dynamically tunable, self-adaptive non-metal-based SERS substrate for multifunctional and high-sensitivity bio-sensing applications.

Results and Discussion

WO 3-x was synthesized by a hydrothermal method. Briefly, tungsten hexachloride (WCl 6 ) was used as the tungsten source. It was dissolved in ethanol, and the solution was transferred to a Teflon-lined stainless steel autoclave and kept at 180 °C for 24 hours. [START_REF] Tian | Synergy of W 18 O 49 and Polyaniline for Smart Supercapacitor Electrode Integrated with Energy Level Indicating Functionality[END_REF] Thereafter, the obtained blue products (WO 3-x ) were collected by centrifugation, washed with ethanol for several times, and finally dried in the air at 60 °C. The morphology of the synthesized WO 3-x was detected by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (see Figure S1, Supporting Information). From the SEM images, it could be found that the prepared WO 3-x nanoparticle shows multiple-tentacles growing on the core plate. The tentacles entangle with each other, and the lengths of these tentacles vary a lot. In the meanwhile, the TEM image reveals more details. For a typical WO 3-x nanoparticle, the size of the core plate is 330 nm×577 nm, and high resolution inspection on the single tentacle further evidences that the lattice constant is 0.378 nm, which belongs to (010) planes of monoclinic WO 3. [START_REF] Cong | Noble Metal-Comparable SERS Enhancement from Semiconducting Metal Oxides by Making Oxygen Vacancies[END_REF] To realize electrical tuning of defect density in the WO 3-x , the synthesized WO 3-x powder was dissolved in ethanol again, sonicated for 15 min and then spin-coated on a Si wafer with prefabricated 100-nm-thick Au planar electrodes. After that, the substrate was annealed at 100 °C for 0.5 hour in the air to improve the adhesion between the WO 3-x and the electrodes (see the Methods). Figure 1a andb show the optical image and schematic structure of the prepared SERS substrate. Principally, WO 3-x deposited between the gap of the two electrodes constitutes the active SERS region, where the defect density could be electrically modulated by applying voltage on the two electrodes.

Figure 1c shows the atomic force microscopy (AFM) topographical image. It can be seen that the width of the electrodes and the spacing between the two electrodes are both 10 μm. Figure 1d shows the cross-section SEM picture of the SERS substrate, it reveals that an 80-nm-thick WO 3-x is deposited between the two electrodes during the spin-coating process. In addition, XRD and UV-vis spectroscopy were measured for the WO 3-x powder sample before and after 100 °C annealing in the air (Figure S2, Supporting Information). It can be seen from the XRD spectroscopy that both of the pristine and annealed WO 3-x are monoclinic phases (P2/m, JCPDS no. 84-1516). Besides, UV-vis spectroscopy discerns slight blue shift of the absorption curve after 100 °C annealing in the air, indicating the partial removing of deep defects, whereas the weak absorption hump is kept at near-infrared region, which reveals the remaining of shallow defects.

Electrical tuning of the defect density was performed using a Keithley 4200-SCS semiconductor parameter analyzer (see the Methods). A voltage was applied on the left electrode and the right one was always grounded. The average electric field between the electrodes could be evaluated as V•cm -1 , where is the applied voltage and is the spacing of the electrodes.

𝑽 𝒅

𝑽 𝒅

Previous understanding revealed that the oxygen vacancy defect density in the WO 3-x layer firmly depended on the leakage current level. [START_REF] Trapatseli | Conductive Atomic Force Microscopy Investigation of Switching Thresholds in Titanium Dioxide Thin Films[END_REF] Typically, conductive filaments constituted by oxygen vacancy chains were created during the voltage sweep phase, and the number of the oxygen vacancies in the conductive filaments were determined by the current level sneaking though the oxide; the higher the leakage current, the higher the density. [START_REF] Wang | Vacancy-Induced Semiconductor-Insulator-Metal Transitions in Nonstoichiometric Nickel and Tungsten Oxides[END_REF] Therefore, to achieve different defect densities in the oxide, the leakage current level in the oxide are precisely controlled with a preset current compliance (I C ) during the voltage sweep. When the leakage current reaches the compliance value, the voltage sweep is immediately aborted, yielding a defect density that corresponds to the preset I C . Figure 2a shows typical current-voltage (I-V) curves obtained on three similar SERS substrates with respective preset I C values. The voltage sweeps were carried out in steps. For the first step (when the WO 3-x was in the pristine state), a relatively low I C (1×10 -7 A) was chosen to prevent catastrophic breakdown of the WO 3-x when the voltage exceeded a certain threshold voltage (> 100 V). Beyond this voltage, the leakage current rose sharply, signaling a rapid increase of defect generation rate. If this increase was not arrested quickly, a thermal runaway situation that led to a catastrophic breakdown of the WO 3-x would occur. Below the threshold voltage, the leakage current was relatively low (< 1×10 -12 A @ 1 V) and was comparable for all three substrates. This showed that the WO 3-x fabricated by the hydrothermal method was of good quality (low defect density) and uniformity. After the first step, the leakage current of each individual SERS substrate was further programed to the preset I C of 1×10 -7 A, 1×10 -5 A, 1×10 -3 A, respectively (inset of Figure 2a). During the entire two-stage electric field programing periods, the O atom of the weak W-O bond was considered to be outstretched away from its lattice site when the electric field gradually increased in the oxide, leaving the oxygen vacancies behind. [START_REF] Gu | The Role of the Disordered HfO 2 Network in the High-κ n-MOSFET Shallow Electron Trapping[END_REF] The continuous accumulation of the oxygen vacancies along the oxide ultimately induced the dielectric breakdown, which brought the current to the compliance of 1×10 -7 A, 1×10 -5 A and 1×10 -3 A, respectively.

The SERS performance of the substrates was evaluated by using the Rhodamine B (RhB) as the Raman reporter (the RhB molecular structure is shown in the inset of Figure 2b). However, before collecting the RhB Raman spectra on the substrates, the Raman spectra of the bare WO 3-x were first discerned (see Figure S3 in Supporting Information). For the WO 3-x powder, Raman peaks that correspond to O-W-O bending and W-O stretching modes are clearly observed, [START_REF] Sungpanich | Large-ScaleSynthesis of WO 3 Nanoplates by a Microwave-Hydrothermal Method[END_REF] whereas no evident peak are found on the 80-nm WO 3-x film (same thickness as that on the prepared substrate). This discrepancy could be ascribed to fewer number of O-W-O bending and W-O stretching that responses to the laser excitation when the film thickness reduces. Thereafter, in a typical experiment of measuring the RhB Raman spectra, after the successful electrical programing of the defect in the oxide, the bias was removed, and then 10 μL of RhB ethanol solution (10 -4 M) was drop-casted on the substrate surface, drying in the atmosphere. The Raman spectra were collected on the WO 3-x region between the two Au electrodes under a 532-nm laser excitation (see the Methods). Figure 2b shows the Raman spectra with the WO 3-x pre-programed to different leakage current levels. Interestingly, no Raman peaks that related to RhB are detected on the pristine WO 3-x within the electrodes gap, implying that the chemical interaction between the substrate material and molecules are very weak under defect deficient condition. On the other hand, for the substrate with the leakage current programed to 1×10 -7 A, it shows that weak Raman signals at 612, 1360, 1507 and 1650 cm -1 (named as P1, P2, P3 and P4) are observed. All of these four Raman peaks are off the WO 3-x ones, confirming the Raman enhancement capability of the substrate for the RhB Raman reporters. Thereafter, with the further increase of the preprogramed leakage current to 1×10 -5 A and 1×10 -3 A, it could be found that the intensities of the above Raman peaks are correspondingly increased. Since the defect densities in the oxide are built upon the leakage current level, the increase of Raman intensity with the arising of the leakage current manifests the magnified CT between the substrate material and molecules.

At this stage, it is worth noting that the Raman intensity enhancement of P4 vibrational mode is obviously higher than those of P1, P2 and P3 vibrational modes (Figure S4, Supporting Information). According to earlier studies, this selective enhancement of specific vibrational mode under the CM condition is determined by the nature of the particular atoms or groups that contributes to the vibrational mode. [START_REF] Centeno | Selection Rules of the Charge Transfer Mechanism of Surface-Enhanced Raman Scattering: The Effect of the Adsorption on the Relative Intensities of Pyrimidine Bonded to Silver Nanoclusters[END_REF] And for a large molecule like RhB, it contains a few vibrational units, thus one vibrational mode always associates with the motions of several vibrational units. In order to uncover the contribution of different vibrational units to a single vibrational mode, previous research works based on density function calculation has been conducted (contribution < 10% is omitted). for the substrate with 1×10 -7 A leakage current can reach 3.01×10 [START_REF] Hu | Surface-Enhanced Raman Scattering Active Gold Nanoparticles with Enzyme-Mimicking Activities for Measuring Glucose and Lactate in Living Tissues[END_REF] . With the further raising of the leakage current levels, the EF can be tuned to higher level, e.g., 3.97×10 5 at 1×10 -5 A and 1.14×10 6 at 1×10 -3 A. Furthermore, to approximate the lower limit of detection (LOD) for the substrates with 10 -3 A leakage current (substrate with highest EF) were also evaluated by decreasing the RhB concentration from 10 -4 to 10 -7 M (see Figure S6 in Supporting Information).

Clearly, P1 and P4 peaks are discernible even the molecular concentration reaches 10 -7 M, indicating the superior performance of the programed substrate for detecting low trace analyst.

Additionally, the SERS spectra of the other Raman reporters such as Rhodamine 6G (R6G),

Crystal Violet (CV) and Methylene Blue (MB) were also collected and shown in Figure S7 of the supporting information. Obviously, the substrate developed in this work also show admirable Raman signal enhancement capability for the above different molecules.

The retention of the EF for the substrate is also evaluated. To achieve this, the programed substrate (10 -3 A) with RhB (10 -4 M) was kept in the dry box (atmospheric condition, room temperature and 20% humidity) for 14 days, and after that, the Raman spectra were collected again. Unfortunately, it could be observed in the Figure 3a and b that there are 42 % drop of P1 peak intensity and about 32% drop of P4 peak intensity, respectively. In an effort to identify the root cause of these evident Raman peak intensity drop, the leakage current of the substrates were measured, and it revealed in the Figure 3c that it reduced significantly. In other words, large components of the defects in the oxide were eliminated by redoxing reaction during the rest period in the air (Figure S8, Supporting Information), therefore the CT efficient was weakened, resulting in the drop of the Raman peak intensities. However, relying on the flexible electrical tuning ability of the designed SERS substrate based on WO 3-x material, it was convenient to reprograming the substrate to the pre-defined current level, as shown in the Figure 3c. The Raman spectra were collected for the refreshed substrate, and the peak intensities (P1 and P4) were extracted as shown in Figure 3a and b again. Apparently, it could be found that P1 and P4 peak intensities resume as the leakage current level recovers, which confirms the role of defects in enhancing the Raman signal. At this stage, it is worth noting that the Raman intensity not only can be in-situ recovered to the original intensity through elevating leakage current level, but it also can be electrically tuned down. Nowadays, progress in SERS technology usually can achieve the LOD to 10 -10 M or even lower. Nevertheless, a high EF may induce signal saturation quickly when the molecule concentration increases, thus restricting the whole detection range, especially for molecules with strong Raman activity. Figure 4a and b illustrates the Raman intensity evolution with the lively changing of the leakage current for the designed WO 3-x -based SERS substrate. First, the initial leakage current level of the substrate was programed to 1×10 -3 A (curve 1, Figure 4a) to achieve the maximum EF, and then 5 μL of 10 -4 M RhB ethanol solution was dropped on the substrate surface, drying for Raman measurement. As it shows in Figure 4b that relatively strong signal intensities of P1 and P4 can be clear observed (curve 1, Figure 4b).

In this situation, a negative voltage sweep was applied on the substrate (curve 2, Figure 4a), which reset the leakage current of the substrate to 1×10 -5 A (curve 3, Figure 4a) due to the partial recovery of the defects. [START_REF] Wang | Vacancy-Induced Semiconductor-Insulator-Metal Transitions in Nonstoichiometric Nickel and Tungsten Oxides[END_REF] The Raman spectra were re-measured on this substrate. Since the EF is adequately determined by the defect density, therefore, in the current condition with fewer defects in the oxide, the intensity of P1 and P4 reduces (curve 3, Figure 4b), which is evidently lower than that when the leakage current is 10 -3 A. The above procedure demonstrates that by readily adjusting the leakage current through the WO 3-x , the same substrate can be adapted to handle different molecule concentrations, thereby potentially extending the detection range that a single substrate can provide. In addition, it is also confirmed that a positive voltage sweep could set the current back to 1×10 -3 A (curve 4, Figure 4c) and restore the initial high EF (curve 4, Figure 4b), enabling its capability to detect the low concentration molecules.

It is well accepted that the Raman enhancement of the semiconductor material is ascribed to the CM. The oxygen vacancy, as the main defect morphology in the imperfect semiconductor material, has helped the CT. [START_REF] Trapatseli | Conductive Atomic Force Microscopy Investigation of Switching Thresholds in Titanium Dioxide Thin Films[END_REF] Herein, to confirm the role of the oxygen vacancy defects for signal enhancement, conductive atomic force microscopy (C-AFM) is used to detect the defect distribution in the WO 3-x . Since in the Raman measurement, the penetration depth of the laser in the WO 3-x material could be several micrometers, thus the electrons could be readily excited to the conduction band by the injected photons even if the defects are situated below the surface, i.e. within the bulk of the WO 3-x , however a surface-sensitive technique such as the C-AFM tip is unable to detect these bulk defects, scanning on the untreated surface only gives the extremely low leakage current (Figure S9, Supporting Information). [START_REF] Johansson | The Importance of Oxygen Vacancies in Nanocrystalline WO 3-x Thin Films Prepared by DC Magnetron Sputtering for Achieving High Photoelectrochemical Efficiency[END_REF] Therefore, chemical thinning of the WO 3-x was necessary to expose the defect clusters. In this work, NH 3 •H 2 O etchant was used to remove a top layer of the WO 3-x before the C-AFM measurement (see the Methods). [START_REF] Mutumi | A New Cleaning Technique for X-Ray Masks in Alkaline Solutions by Direct Control of Electrochemical Potential[END_REF] After the chemical thinning of the oxide layer, C-AFM topography and current mapping were carried out with a 10 V bias applied on the C-AFM tip, and the right Au electrode grounded (inset of Figure 5b). Figure 5a andc show the 3-dimensional topographical profiles of two substrates, one having a pristine WO 3-x and the other pre-programmed to a leakage current of 1×10 -3 A, and Figure 5b and d show the corresponding current maps. The lateral profile of the SERS substrates, comprising the left and right electrodes and the sandwiched WO 3-x (see Figure 1b), are clearly reproduced by the C-AFM topography scan. Additionally, at the right side of the current map of both substrates, a large bright area (corresponding to a saturated leakage current level of 20 nA)

representing the highly conductive right electrode, which distinctly confirms the well-grounded electrodes. In addition, at the left side of the current map, no bright area that represents the electrode could be found for the substrate with 1×10 -12 A leakage current (pristine substrate) on Figure 3b, this is ascribed to the blocked current path between two electrodes in the defect deficient WO 3-x , and thus no leakage current can be detected on the left electrodes. However, this is not the case for the substrate with 1×10 -3 A leakage current, area with saturated current level at the left of the substrate current map still could be clearly observed, indicating the presence of electric field formed conductive path in the oxide (Figure 5d). Furthermore, in the spacing region between the electrodes for the substrate with 1×10 -12 A leakage current, it can be observed that the peak value of the current map is as low as 100 pA only, which is consistent with the electrical measurement. On the other hand, bright shades representing strong leakage paths exposed after chemical thinning could be found on Figure 5d. These high leakage current paths connect the left and right electrodes, comprising the conductive filaments in the sandwiched WO 3-x region, which unambiguously reveals the high density of oxygen vacancy chains in the spacing region.

To further shed light on the mechanism underlying the Raman signal enhancement, firstprinciples simulation was performed. Non-polar WO 3-x slabs with oxygen-atom terminated surfaces were used (Figure 6a). A hybrid density functional that based on the semi-local approximation developed by Perdew, Burke and Ernzerhof (PBE) was implemented to correct the underestimated band gap (see the Methods). To investigate the energy level alignment between the WO 3-x and RhB molecules with the incorporation of oxygen vacancy, the average electrostatic potential (AEP) was calculated and aligned to the vacuum level which was scaled to 0 V, then the valance band maximum (VBM), conduction band minimum (CBM) and defect energy levels were consequently aligned to the AEP based on the band diagram. [START_REF] Alkauskas | Defect Energy Levels in Density Functional Calculations: Alignment and Band Gap Problem[END_REF] It is shown in Figure 6b that the band gap of the WO 3-x given by the simulation is ~3.5 eV, which is consistent with the experimental data and is significantly improved in terms of accuracy as compared to the classical theoretical calculation. [START_REF] Vasilopoulou | Fast Recovery of the High Work Function of Tungsten and Molybdenum Oxides via Microwave Exposure for Efficient Organic Photovoltaics[END_REF] As for the RhB molecules, the highest occupied molecular orbital (HOMO) is at -4.85 eV and the lowest unoccupied molecular orbital (LUMO) is at -1.64 eV.

These calculated molecular orbitals also match the reported results. [START_REF] Zhao | Rhodamine B-Sensitized BiOCl Hierarchical Nanostructure for Methyl Orange Photodegradation[END_REF][START_REF] Gupta | Rhodamine-Derived Highly Sensitive and Selective Colorimetric and Off-on Optical Chemosensors for Cr3+[END_REF] Since the WO 3-x was terminated with un-passivated oxygen atoms, surface states (SS) were introduced into the band gap even in the slab without oxygen vacancy. However, both of them were mid-gap states and the energy intervals between the neighboring states were still sizable, resulting in inefficient CT under the Raman laser irradiation. On the other hand, the oxygen vacancy brought in an extra level below the SS as evidenced in the band diagram of the WO 3-x slab with oxygen vacancy. In the meanwhile, the presence of the oxygen vacancy in the oxide also up-shifted slightly the band edges, giving a better alignment between the CBM of WO 3-x and the HOMO level of the RhB molecule. [START_REF] Liu | Correlation of Oxygen Vacancy Variations to Band Gap Changes in Epitaxial ZnO Thin Films[END_REF] Therefore, it could be expected that those electrons excited under an excitation of 532 nm were more readily transferred to the molecules with the help of the multi-step defect levels. At this stage, it is worth noting that when more oxygen vacancy defects are accumulated in the oxide, additional defect levels are introduced in the band gap. Because of the lower interlevel energy difference, these supplemental energy levels further facilitate the CT, resulting in a greater magnification of the Raman scattering signals.

Conclusion

In summary, an electrically tunable SERS substrate based on the metal oxide is demonstrated.

Experimental investigations have shown that the SERS enhancement factor, from 3.01×10 5 to 1.14×10 [START_REF] Lane | SERS Nanoparticles in Medicine: From Label-Free Detection to Spectroscopic Tagging[END_REF] , can be conveniently tuned through the electrical programing of the defect density.

Moreover, relying on the advantage of in-situ defect density tuning, the self-adaption capability of the substrate to different molecular concentrations is also evidenced, which provides a facile way to enlarge the detection range. More importantly, theoretical calculations based on firstprinciples are also performed, and the results confirm that the defect levels introduced by the oxygen vacancy defects can be well-aligned to the molecular levels, facilitating the CT between the substrate oxide and the molecules. These results not only further enrich the knowledge of the CM, they also pave the way towards the implementation of novel, highly flexible non-metalbased SERS substrates with variable detection capability. All chemicals were used without further purification. In all the experiments, deionized water (resistivity of 18.2 MΩ •cm) was used to prepare the solutions.

Methods

Synthesis of WO 3-x : WO 3-x was synthesized by implementing a simple hydrothermal method.

Basically, WCl 6 (0.099 g) was used as the tungsten source and dissolved in ethanol (30 mL), then the prepared solution was transferred to a Teflon-lined stainless-steel autoclave and kept at 180 °C for 24 hours. Thereafter, the obtained blue products were collected by centrifugation (15000 rpm), washed with ethanol for several times, and finally it was dried in the air at 60 °C. The SEM images of the substrates were acquired by a Hitachi SU-70 system (Hitachi, Japan)

Fabrication of the

under an accelerating voltage of 5 kV. The TEM images were obtained on a JEM-2100F transmission electron microscope (JEOL, Japan). The UV-vis spectra were collected with a spectrometer (TU1901, P-General, Samutprakarn, Thailand). SERS measurements were made by a Raman microscope equipped with a spectrometer (QE Pro, Ocean Optics, USA). A 532-nm semiconductor laser was used as the excitation source, and the diameter of the spot size of the laser on the substrate is 12.5 μm (100× objective lens). The Raman spectra were collected under a laser power of 1mW, and an integration time of 10 s.

First-principles simulation:

The simulation work was performed by Vienna Ab-initio Simulation Package (VASP). [START_REF] Kresse | Ab Initio Molecular Dynamics for Liquid Metals[END_REF] The ultra-soft pseudo-potential and plane-wave expansions of wave functions and potentials were implemented during the calculation. [START_REF] Vanderbilt | Soft Self-Consistent Pseudopotentials in a Generalized Eigenvalue Formalism[END_REF] The exchange correlation energies were treated within the generalized gradient approximation (GGA) of Perdew, Burke and Ernzerhof (PBE). [START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF] Based on the converging tests, the cut-off energy of 400 eV for the plane wave basis and k-space grids of 3×3×1 with the Monkhorst Pack scheme were

  The first major one happens from the highest occupied molecular orbital to the lowest unoccupied xanthene π orbital, resulting in the geometry change on the xanthene ring. The second minor one happens from the lowest unoccupied xanthene π orbital to an unoccupied phenyl π orbital, resulting in the geometry change on the xanthene ring and the phenyl ring.During the Raman measurement, the vibrations of the atoms or groups prefer to mimic the molecular geometries change, consequently bringing in the significant enhanced Raman intensity. Therefore, considering the nature of these four Raman vibrational modes observed on RhB, the intensity of P4 vibrational mode is promoted much more than the other ones. Other than that, the statistical data for the substrates prepared in the same batch are also collected and shown in the FigureS5of the supporting information. Obviously, substrates with similar leakage levels produce comparable Raman intensities, indicating relatively good performance in uniformity of the SERS intensity. In general, the above observations evidently show the viability of electrically tuning the Raman enhancement through defect density control. Besides, the Raman EFs of the substrate were calculated based on the integrated peak intensity at 1650 cm -1 (details in the Supporting Information), and shown in the Figure2c. It could be found that the preliminary EF

Figure 1 .

 1 Figure 1. a) Optical image and b) 3D schematic structure of the SERS substrate, c) two-dimensional AFM topographical image, d) cross-section SEM image of the WO 3-x -based SERS substrate. 557x445mm (72 x 72 DPI)

Figure 2 .

 2 Figure 2. a) Current-voltage (I-V) curves obtained on three similar SERS substrates during the first voltage sweep, b) Raman spectra obtained on a pristine substrate and on the three substrates after programming each of them to a different leakage current, c) SERS enhancement factor as a function of the programmed leakage currents. The inset in (a) shows the second voltage sweep that finally brings the leakage current to the specific compliant current level. The inset in (b) shows the molecular structure of RhB. 916x742mm (72 x 72 DPI)

Figure 3 .

 3 Figure 3. a) The P1 peak intensities measured on the same substrate with three different conditions, b) the P4 peak intensities measured on the same substrate with three different conditions, c) the leakage current levels of the substrate that corresponds to the respective peak intensities in (a) and (b). 707x418mm (72 x 72 DPI)

Figure 4 .

 4 Figure 4. a) Negative voltage sweep that reset the current to lower level. Curve 1 is the initial current level, curve 2 shows the current change when the negative voltage sweep is applied; curve 3 is the current level after negative voltage sweep, b) The evolution of the Raman intensity with in-situ change of the leakage current. The numbers labeled on the respective Raman spectra correspond to the current levels in (a) and (c), c) the leakage current level after reprograming the substrate that has been subjected to the negative voltage sweep. 698x413mm (72 x 72 DPI)

Figure 5 .

 5 Figure 5. SERS substrate after the chemical thinning. a) The three-dimensional AFM topographical image of a SERS substrate with 1×10 -12 A leakage current, b) the corresponding leakage current map obtained on the same substrate in (a), c) the three-dimensional AFM topographical image of the substrate with 1×10 -3 A preprogramed leakage current, d) the corresponding leakage current map obtained on the same substrate in (c). The inset in (b) shows the schematic diagram of the C-AFM measurement. Red arrows show the direction of current flow from the probe towards the grounded right electrode. 533x412mm (72 x 72 DPI)

Figure 6 .

 6 Figure 6. a) Atomic structures of the WO 3-x slabs without oxygen vacancy (left) and with oxygen vacancy (right). In both case, there is a 15-Å thick vacuum layer above the atoms. The large red atoms are oxygen, and the small grey atoms are tungsten, b) alignment of the energy levels of the RhB molecule (center), WO 3-x slabs with(left) and without (right) the oxygen vacancy defect. AEP represents the averaged electrostatic potential. CBM means conduction band minimum and VBM means valance band maximum. LUMO represents the lowest unoccupied molecular orbital and HOMO represents the highest occupied molecular orbital. 573x269mm (72 x 72 DPI)

  [START_REF] Watanabe | DFT Vibrational Calculations of Rhodamine 6G Adsorbed on Silver: Analysis of Tip-Enhanced Raman Spectroscopy[END_REF][START_REF] Jensen | Resonance Raman Scattering of Rhodamine 6G as Calculated Using Time-Dependent Density Functional Theory[END_REF] Based on the comprehensive discussion in ref49 and 50, it can be understood in RhB that P1 vibrational mode (612 cm -1 ) comprises two main components, in which 49% comes from the motion of the xanthene ring and 48% belongs to the motion of the phenyl ring with the COOH group. Similarly, for P2 vibrational mode (1360 cm -1 ), 70% is contributed by the motion of the xanthene ring and the NHC 2 H 6 group provides another 17% contributions. For P3 vibrational mode (1507 cm -1 ), the motion of the xanthene ring produces 69% contributions and NHC 2 H 6 group gives another 25% contributions. Lastly, the motion of the xanthene ring delivers 98% contributions to the P4 vibration mode (1650 cm -1 ). On the other hand, as the CM is intuitively induced by the CT between the molecular orbitals (ground state excited state), and it can be derived from the simulation work indicates that, under the excitation of 532 nm laser, two-electron transition paths usually happen in the RhB.

Materials :

 Materials WCl 6 (99.99 %) was purchased from Macklin. Ethanol (99.7 %), NH 3 •H 2 O (25 %), H 2 SO 4 (95 %), and H 2 O 2 (30 %) were purchased from Sinopharm Chemical Reagent Co., Ltd.

SERS substrate: The

  Si wafer was clean by H 2 SO 4 /H 2 O 2 solution (3: 1), and then a 100-nm thermal SiO 2 was grown in a furnace at 1000 °C. Au (80 nm)/Ti (20 nm) electrodes spaced 10 μm apart were fabricated on the SiO 2 /Si substrate by UV photolithography and electron-beam evaporation of Ti, Au metals, and thereafter followed by a lift-off process. To deposit the synthesized WO 3-x powder on the pre-prepared substrate, the dried WO 3-x powder (5 mg) was dissolved in ethanol (50 mL) again, sonicated for 15 min. Thereafter, 10 μL of the prepared solution was drop-casted on the substrate center, and spin-coated onto the substrate at 2000 rpm for 30 s. Finally, the substrate was annealed at 100 °C for 0.5 hour in an argon gas ambient. Voltage sweep and leakage current measurements were performed using a Keithley 4200-SCS semiconductor parameter analyzer system (Tektronix, US). The topographical profile of the substrate and the leakage current map were analyzed by AFM and C-AFM on a Cypher S AFM system (Oxford Instruments, UK). For the C-AFM measurement, the substrate was treated with 5 % wt. NH 3 •H 2 O at 80 °C for 20 hours in order to remove the surface WO 3-x layer and expose the bulk defects. The XRD spectra were recorded on a D8 Advance diffraction-meter equipped with a LynxEye XE detector (Bruker-AXS, Karlsruhe, Germany).

	Characterization:

  used to calculate the total system energy. For structure optimization, the conjugate gradient method was used and the ion positions were optimized until the residual force was less than 0.01 eV ・Å -1 . Moreover, the hybrid density functionals based on the semi-local PBE approximation were implemented by replacing 20 % of PBE exchange with the exact exchange to correct the underestimated band gap.[START_REF] Heyd | Hybrid Functionals Based on a Screened Coulomb Potential[END_REF] In the simulation work, an oxygen atom terminated non-polar WO 3 slab with 15-Å thick vacuum layer was used. The size of the WO 3 slab is 7.73×7.67×26.67 Å, and it contains a total of 12 tungsten atoms and 40 oxygen atoms. To create an oxygen vacancy, one oxygen atom will be removed and the resultant slab was subjected to the same structural optimization procedure. As for the calculating of RhB molecule, an RhB molecule was put in a cell with the size of 30×30×30 Å. and D. S. Ang would like to acknowledge the computing resource kindly supported by the Singapore's National Supercomputing Center under Project ID 12001379.

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AUTHOR INFORMATION

Corresponding Author guchenjie@nbu.edu.cn; zhoujun@nbu.edu.cn; shuwen.zeng@unilim.fr.