domain: chim phys

text:
Oleg Lupan

Vasile Postica

Thierry Pauporté

Mathias Hoppe

Rainer Adelung

Dr Eng Lupan

Keywords: Au-modified ZnO, nanowire, UV photodetector, nanophotodetector

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Abstract

In this work, ultraviolet (UV) nanophotodetectors based on individual gold-functionalized ZnO nanowires (Au/ZnO NWs) with different diameters and with different Au content were fabricated using a focused ion beam/scanning electron microscopy (FIB/SEM) system. The influence of the Au content, the diameter of the NWs, the applied bias voltage, the temperature and the relative humidity during operation on the UV sensing properties was investigated in detail.

The necessity of a higher Au nanoparticles (NPs) coverage of individual Au/ZnO NWs in order to obtain higher UV response is demonstrated for the first time. A high UV response (IUV/Idark) of 21 for an individual Au/ZnO NW (with ~ 6.4% Au NPs coverage) was obtained, which is by a factor of 17 higher compared to unmodified ZnO NWs. The investigation regarding the NW diameter confirmed that thinner NWs are more suitable for UV sensing applications due to the greater influence of surface phenomena on electrical properties. Most importantly, the elaborated nanophotodetectors in this work, based on individual Au/ZnO NWs, show an extreme low influence caused by water vapors, i.e. relative humidity on UV response, which is critical for practical applications of high performance UV photodetectors in normal ambient conditions. Our experimental results demonstrate clearly that the n-type Au/ZnO NWs with enhanced optoelectronic properties are highly promising building nano-blocks for near future nanooptoelectronic devices and possible for biosensing applications.

Introduction

One-dimensional (1-D) ZnO nanostructures, such as nanowires, nanobelts, nanotubes and nanorods are ideal for building blocks for high-performance electronic, optoelectronic and sensing devices for environmental applications [START_REF] Huang | Room-Temperature Ultraviolet Nanowire Nanolasers[END_REF][START_REF] Lao | ZnO Nanobelt/Nanowire Schottky Diodes Formed by Dielectrophoresis Alignment across Au Electrodes[END_REF][START_REF] Liu | Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles[END_REF][START_REF] Kolmakov | Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles[END_REF][START_REF] Fan | ZnO nanowire field-effect transistor and oxygen sensing property[END_REF][START_REF] Hu | Fast-Response Nanowire Sensors by Using Schottky Contacts[END_REF][START_REF] Li | Oxygen sensing characteristics of individual ZnO nanowire transistors[END_REF][START_REF] Li | Electronic transport through individual ZnO nanowires[END_REF][START_REF] Li | Nanowire electronic and optoelectronic devices[END_REF]. In the case of sensing devices (e.g. UV photodetectors and gas sensors) the high influence of surface phenomena, such as adsorption/desorption of gaseous species on the electrical transport through a restricted conduction channel of 1-D nanostructures paves the way to fabricate highly sensitive UV photodetectors and gas sensors based on individual nanostructures [START_REF] Liu | Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles[END_REF][START_REF] Li | Oxygen sensing characteristics of individual ZnO nanowire transistors[END_REF][START_REF] Li | Electronic transport through individual ZnO nanowires[END_REF][START_REF] Lupan | Highly sensitive and selective hydrogen single-nanowire nanosensor[END_REF][START_REF] Lupan | Silver-doped zinc oxide single nanowire multifunctional nanosensor with a significant enhancement in response[END_REF][START_REF] Lupan | Functionalized Pd/ZnO Nanowires for Nanosensors[END_REF][START_REF] Lupan | Enhanced UV and ethanol vapour sensing of a single 3-D ZnO tetrapod alloyed with Fe2O3 nanoparticles[END_REF][START_REF] Lupan | Individual hollow and mesoporous aero-graphitic microtube based devices for gas sensing applications[END_REF][START_REF] Lupan | Properties of a single SnO2:Zn2SnO4 -Functionalized nanowire based nanosensor[END_REF]. The main advantage of UV photodetectors based on individual ZnO NWs is the high internal gain, which is attributed to the presence of oxygen-related hole-trap states at the NW surface which prevents charge-carrier recombination through prolongation of the photo-carrier lifetime [START_REF] Soci | ZnO Nanowire UV Photodetectors with High Internal Gain[END_REF].

Different methods were reported in order to considerably enhance the UV sensing properties of individual ZnO NWs, such as doping [START_REF] Lupan | Silver-doped zinc oxide single nanowire multifunctional nanosensor with a significant enhancement in response[END_REF], surface functionalization with noble metals [START_REF] Liu | Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles[END_REF], formation of individual Schottky contacts at one end of the NW [START_REF] Hu | Fast-Response Nanowire Sensors by Using Schottky Contacts[END_REF], as well as functionalization with other metal oxide NPs [START_REF] Lupan | Enhanced UV and ethanol vapour sensing of a single 3-D ZnO tetrapod alloyed with Fe2O3 nanoparticles[END_REF], and the utilization of the piezo-phototronic effect [START_REF] Yang | Enhancing Sensitivity of a Single ZnO Micro-/Nanowire Photodetector by Piezo-phototronic Effect[END_REF]. For example, Hu et al. fabricated UV photodetectors based on a ZnS/ZnO biaxial nanobelt with high spectral selectivity and wide-range photoresponse in the UV-A band [START_REF] Hu | An Optimized Ultraviolet-A Light Photodetector with Wide-Range Photoresponse Based on ZnS/ZnO Biaxial Nanobelt[END_REF]. Lao et al. demonstrated that the UV response of individual ZnO nanobelts can be enhanced by almost 5 orders of magnitude by surface functionalizing with a polymer that has a high UV absorption ability [START_REF] Lao | Giant Enhancement in UV Response of ZnO Nanobelts by Polymer Surface-Functionalization[END_REF]. While the UV sensing properties of photodetectors based on Au-modified ZnO microand nanostructures have been reported many works [START_REF] Gogurla | Multifunctional Au-ZnO Plasmonic Nanostructures for Enhanced UV Photodetector and Room Temperature NO Sensing Devices[END_REF][START_REF] Jin | High-performance flexible ultraviolet photoconductors based on solution-processed ultrathin ZnO/Au nanoparticle composite films[END_REF], only the several results were reported based on individual Au-modified ZnO structures [START_REF] Liu | Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles[END_REF][START_REF] Pescaglini | Hot-Electron Injection in Au Nanorod-ZnO Nanowire Hybrid Device for Near-Infrared Photodetection[END_REF]. For example, Liu et al. fabricated the UV photodetectors based on an individual ZnO NW with and without Au NPs and observed that surface-functionalization with Au NPs can drastically reduce the response time of ZnO nanowire photodetectors [START_REF] Liu | Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles[END_REF].

However, the interdependence of such important parameters as surface coverage with Au NPs and the diameter of the ZnO NW, relative humidity, applied bias and operation temperature were not investigated and reported. In this work, we integrated individual Au-modified ZnO NWs with different Au content and different diameters using a dual beam FIB/SEM system. The Au/ZnO NWs were synthesized in a one-step electrochemical deposition [START_REF] Lupan | Synthesis of Au-modified ZnO NWs for ultra-sensitive gas nanosensor[END_REF]. Besides the report about the influence of the already enumerated factors, i.e. diameter, relative humidity, applied bias voltage and temperature for a single Au/ZnO NWs based device, the corresponding physical model and the mechanism of detection are tentatively proposed and discussed.

Experimental part

The Au/ZnO NW arrays were grown on glass sheet substrates coated with polycrystalline F-doped SnO2 (FTO) having a resistance of 10 Ω/ (used as a working electrode) by electrochemical deposition in a classical three-electrode electrochemical cell [START_REF] Lupan | Silver-doped zinc oxide single nanowire multifunctional nanosensor with a significant enhancement in response[END_REF][START_REF] Pauporté | Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode[END_REF]. The growth solution contained 0.2 mM ZnCl2, 0.1 M KCl as supporting electrolyte and was subjected to continuous bubbling of oxygen in a bath solution [START_REF] Pauporté | Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode[END_REF][START_REF] Belghiti | Mechanistic study of ZnO nanorod array electrodeposition[END_REF]. For variation of Au NPs content on the surface of ZnO NWs, different concentrations of HAuCl4 (Sigma-Aldrich, >99.9%) were utilized (from 0.3 to 2.0 μM). Upon deposition, the substrate was rotated at a constant speed of ω = 300 rotations/min (rpm) [START_REF] Lupan | Silver-doped zinc oxide single nanowire multifunctional nanosensor with a significant enhancement in response[END_REF][START_REF] Pauporté | Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode[END_REF]. More details on the electrochemical deposition of doped ZnO NWs are presented in previous works [START_REF] Lupan | Silver-doped zinc oxide single nanowire multifunctional nanosensor with a significant enhancement in response[END_REF][START_REF] Pauporté | Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode[END_REF][START_REF] Lupan | Hydrothermal treatment for the marked structural and optical quality improvement of ZnO nanowire arrays deposited on lightweight flexible substrates[END_REF]. The detailed morphological, structural, chemical and optical properties of Au/ZnO NW arrays were performed in the previous work [START_REF] Lupan | Synthesis of Au-modified ZnO NWs for ultra-sensitive gas nanosensor[END_REF] and indicated that Au ions are not incorporated in the ZnO matrix (i.e. no doping was observed or proved) but Au agglomerations are deposited on the surface of the ZnO NWs in the form of nanoclusters with a diameter in the range of 20 -50 nm [START_REF] Lupan | Synthesis of Au-modified ZnO NWs for ultra-sensitive gas nanosensor[END_REF]. In other words, the growth and surface functionalization of ZnO NWs with Au nanoparticles is performed with the same electrochemical deposition process, which is an important advantage because it allows to cut down additional technological steps for post-growth surface functionalization [START_REF] Lupan | Synthesis of Au-modified ZnO NWs for ultra-sensitive gas nanosensor[END_REF].

The nanophotodetectors based on individual ZnO:Au NWs were fabricated using the method developed by Lupan et al. in a dual beam FIB/SEM scientific instrument [START_REF] Lupan | Silver-doped zinc oxide single nanowire multifunctional nanosensor with a significant enhancement in response[END_REF][START_REF] Lupan | Synthesis of Au-modified ZnO NWs for ultra-sensitive gas nanosensor[END_REF][START_REF] Lupan | Fabrication of ZnO nanorod-based hydrogen gas nanosensor[END_REF][START_REF] Lupan | Single and networked CuO nanowires for highly sensitive p-type semiconductor gas sensor applications[END_REF].

The UV sensing measurements were performed at room temperature (RT, ~ 25 ºC) in normal ambient air (relative humidity, RH ~ 20%). The UV light irradiance (λ = 365 nm) was set to ~ 1 mW/cm 2 .

Results and discussions

Nanophotodetectors based on an individual Au/ZnO NW. Influence of Au NPs coverage

A typical SEM image of a nanophotodetectors fabricated using an individual Au/ZnO nanowire is presented in Figure 1a. The individual Au/ZnO NW, which is placed on SiO2/Si substrate, is connected to Au/Cr electrodes using two Pt contacts. The currentvoltage (I-V) characteristics of individual ZnO NWs with different Au NPs coverage (determined from SEM images of the devices and calculated by (SNP • 100%)/SNW, where SNP is the Schottky contact area formed by Au NPs (πr 2 , where r is the Au NPs radius) and SNW is the surface of the ZnO NWs (2πrh, where r and h are the radius and length of the NW) and with a 120 ± 10 nm diameter is presented in Figure 1b. The I-V measurements clearly show a resistance decrease of the devices with higher Au coverage. This can be attributed to the formation of the larger Schottky contact area, and respectively to the more narrowed conduction channel [START_REF] Liu | Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles[END_REF][START_REF] Gogurla | Multifunctional Au-ZnO Plasmonic Nanostructures for Enhanced UV Photodetector and Room Temperature NO Sensing Devices[END_REF][START_REF] Feng | In-situ generation of highly dispersed Au nanoparticles on porous ZnO nanoplates via ion exchange from hydrozincite for VOCs gas sensing[END_REF].

Figure 2a shows the room temperature UV photoresponse for individual Au/ZnO NWs with different Au NP coverage (determined from SEM images of the devices and calculated by (SNP • 100%)/SNW, where SNP is the Schottky contact area formed by AuNPs (πr 2 , where r is the Au NPs radius) and SNW is the surface of the ZnO NWs (2πrLNW, where rNW and LNW are the radius and length of the NW)). The measurements were performed under 2 V bias voltage applied to the individual Au/ZnO NW. The SEM images of the nanodevices are shown in our previous work [START_REF] Lupan | Synthesis of Au-modified ZnO NWs for ultra-sensitive gas nanosensor[END_REF]. The content of Au NPs was mainly varied using different concentrations of HAuCl4 in the electrolyte solution. From a previous work it was observed that by increasing the HAuCl4 from 0.3 to 2.0 μM, the concentration of Au NPs on the surface of the Au/ZnO NW arrays increased [START_REF] Lupan | Synthesis of Au-modified ZnO NWs for ultra-sensitive gas nanosensor[END_REF]. However, in the Au/ZnO NW arrays grown using the same content of HAuCl4 (for example 0.9 μM), the Au/ZnO NW with a large variation of Au NPs on the surface can be found (even without Au NPs) [START_REF] Lupan | Synthesis of Au-modified ZnO NWs for ultra-sensitive gas nanosensor[END_REF]. Therefore, the number of Au NPs on the integrated individual Au/ZnO NW depends more on the chosen NW for integration, than on the HAuCl4 content used for growth of these NWs. This is the reason why the UV response was plotted against the Au NPs coverage and not against the HAuCl4 concentration used for growth of Au/ZnO NWs.

The UV response for Au NPs coverage of 0, ~ 1.2, ~ 1.5, ~ 3.5 and ~ 6.4 % is ~ 1.2, ~ 2.75, ~ 5, ~ 9 and ~ 21, respectively. As can be observed, the UV response increases with Au NPs coverage. Because the diameter of all NWs is about 120 ± 10 nm, the surface of the formed Schottky contacts at the Au/ZnO interface plays an important role for the determination of the device parameters [START_REF] Kolmakov | Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles[END_REF][START_REF] Gogurla | Multifunctional Au-ZnO Plasmonic Nanostructures for Enhanced UV Photodetector and Room Temperature NO Sensing Devices[END_REF]. Thus, by increasing the surface of the Schottky contact, the higher influence of surface phenomena on the charge transport of an individual Au/ZnO NW can be achieved [START_REF] Hernandez-Ramirez | On the role of individual metal oxide nanowires in the scaling down of chemical sensors[END_REF][START_REF] Hernandez-Ramirez | Insight into the Role of Oxygen Diffusion in the Sensing Mechanisms of SnO2 Nanowires[END_REF].

Figure 2b shows the dynamic response to UV light of Au/ZnO NWs without Au NPs on the surface and with ~ 1.2% and ~ 6.4% Au NPs coverage. The rise and decay time constants of the photocurrent are determined through a bi-exponential fit respectively from the corresponding time-resolved measurements, as was reported previously [START_REF] Gedamu | Rapid Fabrication Technique for Interpenetrated ZnO Nanotetrapod Networks for Fast UV Sensors[END_REF]:

        - +         - + = - - 2 1 1 1 ) ( 2 1 r r t t dark e A e A I t I   (1) 2 1 4 3 ) ( d d t t dark e A e A I t I   - - + + = (2)
where A1, A2, A3 and A4 are positive constants. τr1, τr2 and τd1, τd2 are time constants for rising and decaying of the photocurrent, respectively. The results of the curve fittings are presented in Figure 2c. τr1 and τd1 are correlated to the rapid concentration change of the charge carriers when the UV light is switched on/off [START_REF] Postica | Multifunctional device based on ZnO:Fe nanostructured films with enhanced UV and ultra-fast ethanol vapour sensing[END_REF][START_REF] Zhang | Adsorption and photodesorption of oxygen on the surface and crystallite interfaces of sputtered ZnO films[END_REF]. τr2 and τd2 dependent of trapped carriers and their release due to vacancies of oxygen defects in the thin film, as well as photodesorption/adsorption of oxygen molecules, which are in comparison much slower processes [START_REF] Postica | Multifunctional device based on ZnO:Fe nanostructured films with enhanced UV and ultra-fast ethanol vapour sensing[END_REF][START_REF] Zhang | Adsorption and photodesorption of oxygen on the surface and crystallite interfaces of sputtered ZnO films[END_REF][START_REF] Postica | Morphology dependent UV photoresponse of Sn-doped ZnO microstructures[END_REF]. From Figure 2c it can be observed that by an increase in Au NP coverage of individual Au/ZnO NWs, the time constants of rising and decaying photocurrents are considerably decreased. This is more observable from The improved UV sensing properties of individual Au/ZnO NWs compared to a ZnO NW can be explained as follows. The UV sensing performances of micro-and nanostructures of metal oxides depend on electronhole (e --h + ) pairs generation and surface processes, such as adsorption/photodesorption of oxygen molecules [START_REF] Lupan | Properties of a single SnO2:Zn2SnO4 -Functionalized nanowire based nanosensor[END_REF][START_REF] Dhara | Improved fast photoresponse from Al doped ZnO nanowires network decorated with Au nanoparticles[END_REF][START_REF] Paulowicz | Zinc oxide nanotetrapods with four different arm morphologies for versatile nanosensors[END_REF]. In the dark, oxygen molecules (O2) are adsorbed on the surface of the ZnO NW by capturing free electrons (O2 + e -→ O2 -) [START_REF] Liu | Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles[END_REF][START_REF] Postica | Schottky Diode Based on a Single Carbon-Nanotube-ZnO Hybrid Tetrapod for Selective Sensing Applications[END_REF][START_REF] Gröttrup | UV detection properties of hybrid ZnO tetrapod 3-D networks[END_REF]. In result, a surface electron depletion region (Ldark) is formed with a higher resistivity compared to the conduction channel (ddark, see Figure 2d). In this case, the electrical conductance of the ZnO NW (G) is given by [START_REF] Prades | The effects of electron-hole separation on the photoconductivity of individual metal oxide nanowires[END_REF]:

  NW dark NW L L r n q G 2 0 - =   ( 3
)
where q is the electron charge, n0 is the concentration of free electrons and µ is the electron mobility.

Under UV illumination, e --h + pairs are photogenerated [START_REF] Lupan | Silver-doped zinc oxide single nanowire multifunctional nanosensor with a significant enhancement in response[END_REF]. While the eincreases the photocurrent through the conduction channel of the NW, the h + migrates to the ZnO surface and discharges O2 -by surface electron-hole recombination (h + + O2 -→ O2), which leads to the narrowing of the surface electron depletion region (LUV) and a widening of the conduction channel (dUV, see Figure 2f) [START_REF] Lupan | Silver-doped zinc oxide single nanowire multifunctional nanosensor with a significant enhancement in response[END_REF][START_REF] Soci | ZnO Nanowire UV Photodetectors with High Internal Gain[END_REF]. The photoresponse in a steady state (ΔGph, when recombination and generation rates (g) are equal) is given by [START_REF] Prades | The effects of electron-hole separation on the photoconductivity of individual metal oxide nanowires[END_REF]:

surf bulk ph g p n G   / 1 / 1 + =  =    (4)
where Δn and Δp are the excess numbers of n and p carriers, and τbulk and τsurf are the lifetimes of the photocarriers recombined in the bulk and at the surface.

In the case of Au/ZnO NWs, the presence of Au NPs on the surface leads to a formation of Schottky barriers at the Au/ZnO NW interface due to a higher work function of Au (φAu = 5.1 eV) compared to those of ZnO (φAu = 4.1 eV) (see Figure 2e) [START_REF] Liu | Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles[END_REF][START_REF] Joondong | ZnO nanowire-embedded Schottky diode for effective UV detection by the barrier reduction effect[END_REF]. This results in a much more narrowed conduction channel of the Au/ZnO NW, and a lower dark current, which was observed in Figure 1b [START_REF] Liu | Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles[END_REF][START_REF] Lupan | Silver-doped zinc oxide single nanowire multifunctional nanosensor with a significant enhancement in response[END_REF][START_REF] Soci | ZnO Nanowire UV Photodetectors with High Internal Gain[END_REF]. Under UV illumination, the photogenerated holes will migrate to the Au NPs due to the interactive Coulomb force [START_REF] Liu | Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles[END_REF]. Some of the holes are trapped at the Au-ZnO interface and some of the holes pass through the Schottky barrier at the interface, where the electron-hole recombination will induce a reduction of the Schottky barrier and a narrowing of the electron depletion region, respectively (Figure 2g) [START_REF] Liu | Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles[END_REF]. Also, the presence of Au NPs can improve the light absorption efficiency by particle-induced light scattering, i.e. increasing the number of photogenerated e --h + pairs [START_REF] Atwater | Plasmonics for improved photovoltaic devices[END_REF], as well as increase the lifetime of photogenerated carriers and therefore the photoresponse by the increasing in the efficiency of charge carrier separation (see eq. ( 4)) [START_REF] Prades | The effects of electron-hole separation on the photoconductivity of individual metal oxide nanowires[END_REF][START_REF] He | Photogenerated Charge Carriers and Reactive Oxygen Species in ZnO/Au Hybrid Nanostructures with Enhanced Photocatalytic and Antibacterial Activity[END_REF]. Therefore, the particle-induced scattering and Au/ZnO Schottky barriers can be responsible for the great enhancement of UV sensing properties of an individual Au/ZnO NWbased nanophotodetector [START_REF] Liu | Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles[END_REF].

Influence of the nanowire diameter on sensing performances

The sensing mechanism of individual ZnO micro-and nanostructures is primarily surface related, therefore surface states, oxygen adsorption quantities and the diameter play important roles on the UV sensitivity [START_REF] Liu | Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles[END_REF][START_REF] Liao | Size Dependence of Gas Sensitivity of ZnO Nanorods[END_REF], the influence of the latter for an individual Au/ZnO NW was also investigated.

Influence of the operating temperature and the applied bias voltage on sensing performances

Next, the influence of the operating temperature and the applied bias on the UV sensing properties is investigated. Figure 4a shows the currentvoltage (I-V) characteristic of individual

Influence of the relative humidity on sensing performances

Because the UV sensing properties of UV photodetectors, such as the UV response and the time constants (response/recovery), are dominated by surface phenomena/processes involving adsorption/desorption of oxygen species, other chemical species in the environment reduce or increase the response/recovery time [START_REF] Lai | Effects of humidity on the ultraviolet nanosensors of aligned electrospun ZnO nanofibers[END_REF]. In particular, water molecules are present in high quantities in the environment and can affect the sensing performances of ZnO micro-and nanostructures. It remains a serious limitation for practical photosensing applications [START_REF] Postica | Morphology dependent UV photoresponse of Sn-doped ZnO microstructures[END_REF][START_REF] Lai | Effects of humidity on the ultraviolet nanosensors of aligned electrospun ZnO nanofibers[END_REF][START_REF] Ruey-Chi | The evolution of well-aligned amorphous carbon nanotubes and porous ZnO/C core-shell nanorod arrays for photosensor applications[END_REF].

Therefore, without surface passivation the effect of water vapors is hard to avoid. In this context, the humidity influence on UV sensing properties of individual ZnO nanostructures was less reported [START_REF] Chai | Fabrication and characterization of an individual ZnO microwire-based UV photodetector[END_REF].

Our previous measurements were performed at a normal ambient RH of ~ 20% and we also The low impact of water vapors on the UV sensing properties of an individual Au/ZnO NW can be explained as follows. In presence of water vapors the current level in the dark is slightly increased and under illumination significantly decreased [START_REF] Ruey-Chi | The evolution of well-aligned amorphous carbon nanotubes and porous ZnO/C core-shell nanorod arrays for photosensor applications[END_REF]. These current modifications are attributed to the adsorption of water molecules (H2O) on the surface of ZnO by generation of free electrons [START_REF] Postica | Morphology dependent UV photoresponse of Sn-doped ZnO microstructures[END_REF][START_REF] Tai | Humidity sensing behaviors of nanocrystalline Al-doped ZnO thin films prepared by sol-gel process[END_REF]:

( ) - • • + + -  + + e V Zn OH Zn O O H O Zn O 2 2 2 2 (3)
where VO •• is the vacancy created at an oxygen site, OO is the oxygen atom from the lattice and ZnZn is the zinc atom from the lattice. Therefore, by increasing the concentration of water molecules, more adsorption sites on the ZnO NW surface are occupied by hydroxyl groups and increase the value of the dark current by narrowing the width of the electron depletion region [START_REF] Postica | Morphology dependent UV photoresponse of Sn-doped ZnO microstructures[END_REF][START_REF] Tai | Humidity sensing behaviors of nanocrystalline Al-doped ZnO thin films prepared by sol-gel process[END_REF][START_REF] Li | Competitive surface effects of oxygen and water on UV photoresponse of ZnO nanowires[END_REF]. This reduces the lifetime of photogenerated carriers leading to a lower photocurrent [START_REF] Postica | Morphology dependent UV photoresponse of Sn-doped ZnO microstructures[END_REF][START_REF] Tai | Humidity sensing behaviors of nanocrystalline Al-doped ZnO thin films prepared by sol-gel process[END_REF][START_REF] Li | Competitive surface effects of oxygen and water on UV photoresponse of ZnO nanowires[END_REF]. More details about the mechanism are presented in our previous work [START_REF] Postica | Morphology dependent UV photoresponse of Sn-doped ZnO microstructures[END_REF]. However, in our case the lack of changes in the dark-and photocurrent may indicate on that Au NPs act as hydroxyl adsorbers on the surface of NPs, thereby providing an oxygen adsorption site for surface reactions in a humid atmosphere [START_REF] Suematsu | Surface-modification of SnO2 nanoparticles by incorporation of Al for the detection of combustible gases in a humid atmosphere[END_REF]. The same effect was observed for NiO-doped SnO2 [START_REF] Kim | The Role of NiO Doping in Reducing the Impact of Humidity on the Performance of SnO2-Based Gas Sensors: Synthesis Strategies, and Phenomenological and Spectroscopic Studies[END_REF],

Sb-doped SnO2 [START_REF] Suematsu | Antimony-Doped Tin Dioxide Gas Sensors Exhibiting High Stability in the Sensitivity to Humidity Changes[END_REF], CuO-loaded SnO2 hollow spheres [START_REF] Choi | Ultraselective and ultrasensitive detection of H2S in highly humid atmosphere using CuO-loaded SnO2 hollow spheres for real-time diagnosis of halitosis[END_REF], Al-doped SnO2 NPs [START_REF] Suematsu | Surface-modification of SnO2 nanoparticles by incorporation of Al for the detection of combustible gases in a humid atmosphere[END_REF] and Fe2O3functionalized ZnO tetrapods [START_REF] Lupan | Enhanced UV and ethanol vapour sensing of a single 3-D ZnO tetrapod alloyed with Fe2O3 nanoparticles[END_REF].

Figure 6 shows the schematic model of hydroxyl and oxygen co-adsorption on the surface of ZnO and Au/ZnO NW. In the presence of humidity the hydroxyl groups are adsorbed on the surface of the ZnO NW resulting in width changes of the conduction channel Lair (see Figure 6a,b). By the increase in concentration of water vapors, the adsorption of oxygen species is highly decreased due to hydroxyl poisoning, which was confirmed by other investigations [START_REF] Suematsu | Surface-modification of SnO2 nanoparticles by incorporation of Al for the detection of combustible gases in a humid atmosphere[END_REF][START_REF] Kim | The Role of NiO Doping in Reducing the Impact of Humidity on the Performance of SnO2-Based Gas Sensors: Synthesis Strategies, and Phenomenological and Spectroscopic Studies[END_REF], and extends the width of the conduction channel (Lair+RH, see Figure 6a,b). Thus, less photogenerated holes under UV illumination will migrate to the ZnO surface to discharge O2by surface electron-hole recombination, leading to the lowering in modulation of the conduction channel and respectively to lowering in the UV response (see Figure 6c) [START_REF] Lupan | Silver-doped zinc oxide single nanowire multifunctional nanosensor with a significant enhancement in response[END_REF][START_REF] Soci | ZnO Nanowire UV Photodetectors with High Internal Gain[END_REF].

In the case of a Au/ZnO NW (see Figure 6d,e), the hydroxyl groups are adsorbed mainly on the surface of Au NPs, maintaining the adsorption of O2 -even at high water vapors concentration (in our case even at 80%). Thus, we believe that Au NPs act as hydroxyl absorbers by preferential capture [START_REF] Suematsu | Surface-modification of SnO2 nanoparticles by incorporation of Al for the detection of combustible gases in a humid atmosphere[END_REF][START_REF] Kim | The Role of NiO Doping in Reducing the Impact of Humidity on the Performance of SnO2-Based Gas Sensors: Synthesis Strategies, and Phenomenological and Spectroscopic Studies[END_REF]. In result, the high modulation of the conduction channel under UV illumination is maintained and even increased (Figure 6f), as was already described.

Conclusions

In summary, the individual Au-functionalized ZnO NWs were successfully integrated into nanophotodetector devices using a dual beam FIB/SEM system. The electrodeposition of a high number of Au NPs on the ZnO NW surface leads to the formation of Schottky contacts which in turn improves the UV sensing performances. In particular, an UV response (S = IUV/Idark) of about 21 was obtained for Au/ZnO NWs, which is higher by a factor of 17 compared to unmodified ZnO NWs, demonstrating the high efficiency of Au NPs-surface functionalization of individual ZnO NWs. The investigation of the diameter influence on the UV response revealed that the higher UV response of thinner Au/ZnO NWs is based on the higher influence of adsorption/photodesorption of oxygen species on electrical properties. Other factors that can decrease the UV sensing properties of individual Au/ZnO NWs are higher temperatures and applied bias voltages (which can be also attributed to rise in temperature due to the self-heating effect). In this case the decrease in the UV sensing properties was attributed to the increased dark current due to an enhancement in tunneling currents and the increased probability of recombination of photogenerated e -h + pairs. A physical model for the sensing mechanism based on the band energy theory was proposed to show the origin of the improvement of the UV detection performance for the Au/ZnO nanowire

Figure S1a ,

 S1a Figure S1a, where the normalized UV response of Au/ZnO NWs from Figure2bis presented. In

  photocarrier relaxation behavior is dominated by surface band bending[START_REF] Chen | Photocarrier Relaxation Behavior of a Single ZnO Nanowire UV Photodetector: Effect of Surface Band Bending[END_REF]. It can explain the

Figure 5a .

 5a Figure 5a. The data are presented using the measured current, in order to better demonstrate the

Figure 1 .

 1 Figure 1. (a) SEM image of a nanophotodetector based on an individual Au/ZnO NW with a

Figure 2 .

 2 Figure 2. (a) Measured UV response of individual Au/ZnO NWs (with D = 120 nm) versus Au NPs coverage on the surface of NW. (b) Dynamic UV response of individual Au/ZnO NWs with different Au NPs coverage on the surface. (c) The calculated time constants of rising and decaying photocurrent for individual Au/ZnO NWs. The UV sensing mechanism of individual ZnO NW and Au/ZnO NW: ZnO NW in the dark (d) and under UV illumination (f); Au/ZnO NW in the dark (e) and under UV illumination (g).

Figure 3 .

 3 Figure 3. (a) UV response of individual Au/ZnO NWs versus diameter of the nanowire integrated in nanosensor. (b) Dynamic UV response of individual Au/ZnO NWs with different diameters. (c) The calculated time constants for rising and decaying photocurrent of individual Au/ZnO NWs with different diameters. (d) The UV sensing mechanism based on different diameter (bigger and smaller one) of an Au/ZnO NW used in nanosensors in cross-sectional view.

Figure 4 .

 4 Figure 4. (a) Currentvoltage characteristic of an Au/ZnO NW with diameter of 120 nm at different operating temperatures. (b) UV response versus applied bias voltage and operating temperature. Dynamic UV response at different (c) applied bias voltage and (d) operating temperatures.

Figure 5 .

 5 Figure 5. (a) Dynamic UV response of Au/ZnO NWs with diameter of 190 nm at different values of relative humidity. (b) The calculated time constants for rising and decaying photocurrent. (c) Dynamic response to multiple pulses at 80% RH. (d) UV response at different values of RH for individual Au/ZnO NWs with diameter of 190 nm and different content of Au NPs on the surface. (d) UV response at different values of RH (20, 60 and 90%) for individual Au/ZnO NW with diameter of 190 nm and with different Au NPs coverage on the surface.

Figure 6 .

 6 Figure 6. Schematic model of hydroxyl and oxygen molecules co-adsorption on the surface of: (a) ZnO NW and (b) Au/ZnO NW.

Acknowledgments

Dr. Lupan gratefully acknowledges PSL University, Chimie-ParisTech for invited professor position in 2018 and 2019, CNRS Council for support as expert scientist at IRCP Chimie ParisTech, Paris. This research was sponsored partially by the German Research Foundation (DFG) under the schemes PAK 902 (KI 1263/14-1 & AD 183/16-1) and by Project SFB859. This research was partly supported by the Technical University of Moldova.

Au/ZnO NWs (with ~ 6.4% coverage) with a diameter of 120 nm at different operating temperatures [START_REF] Belghiti | Mechanistic study of ZnO nanorod array electrodeposition[END_REF]70, 100 and 150 ºC). A typical semiconductor I-V characteristic, i.e. increasing of the current value at higher temperatures due to thermal activation of the charge carriers [START_REF] Lao | ZnO Nanobelt/Nanowire Schottky Diodes Formed by Dielectrophoresis Alignment across Au Electrodes[END_REF][START_REF] Li | Electronic transport through individual ZnO nanowires[END_REF], is found. The dependence of the UV response on the applied bias and operating temperature is presented in Figure 4b. It is observed that the highest UV response is obtained at 2 V (~ 21), while a further increase in bias voltage to 4 V and 6 V leads to a response decrease to 5 and 1.1, respectively. At an applied bias voltage of 1 V the UV response is much lower, ~ 3. The reduction in the UV sensing performance at elevated operating temperatures can be explained mainly by the higher dark current due to an enhancement in tunneling currents [START_REF] Shafiei | Reversed bias Pt/nanostructured ZnO Schottky diode with enhanced electric field for hydrogen sensing[END_REF] and the increased probability of recombination of photogenerated e -h + pairs [START_REF] Guo | High responsivity ZnO nanowires based UV detector fabricated by the dielectrophoresis method[END_REF]. The decreasing in the UV response by increasing applied bias voltages can be explained based on the well-known self-heating effect [START_REF] Monereo | Self-heating effects in large arrangements of randomly oriented carbon nanofibers: Application to gas sensors[END_REF][START_REF] Strelcov | Gas Sensor Based on Metal-Insulator Transition in VO2 Nanowire Thermistor[END_REF]. Therefore, the higher the applied bias, the more heat will be generated by Joule dissipation [START_REF] Prades | The effects of electron-hole separation on the photoconductivity of individual metal oxide nanowires[END_REF]. Strelcov et al. assumed that the temperature of the self-heated suspended NW has a linear gradient from highest temperature in the center to the ambient temperature at the electrical leads and that the Joule power, generated in such a NW, is dissipated via three channels: heat transfer to the ambient gas, to the metal contacts and via radiation losses [START_REF] Strelcov | Gas Sensor Based on Metal-Insulator Transition in VO2 Nanowire Thermistor[END_REF]. However, in our case the Au/ZnO NWs are placed on SiO2 (300 nm)/Si substrates, and a part of the temperature is dissipated through this substrate.

As a future perspective, the self-heating effect by Joule dissipation could be improved using suspended NWs.

nanophotodetector. The main advantage of such individual Au/ZnO NWs is the lower influence of water vapors on the UV response, which is very important for practical applications.

Supporting


Geoffroy Kremer

Kejing Zhu

Thomas Pierron

Vincent Fournee

Julian Ledieu

Stéphane Andrieu

Bertrand Kierren

Luc Moreau

Daniel Malterre

Ke He

Qi-Kun Xue

Yannick Fagot-Revurat
email: yannick.fagot@univ-lorraine.fr

Yuan Lu
email: yuan.lu@univ-lorraine.fr

Recovery of surface state bands after desorption of Te capping layer on (Bi1-xSbx)2Te3 ternary topological insulators

Keywords: topological insulator, surface state bands, Dirac point, angle resolved photoemission spectroscopy, capping and desorption

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

Introduction

Discovery of the new quantum state of matter called topological insulators (TI) has attracted world-wide interest due to their exotic properties which are manifestations of a non-trivial band topology [START_REF] Qi | Topological insulators and superconductors[END_REF][START_REF] Vergniory | A complete catalogue of high-quality topological materials[END_REF]. TIs have insulating bulk and conducting edges due to the presence of peculiar surface state bands (SSBs). These SSBs are spin non-degenerated with a unique property of spin momentum locking which is resulted from the strong spin-orbit coupling (SOC) effects in combination with the time reversal symmetry [START_REF] Pedram | Topological surface states protected from backscattering by chiral spin texture[END_REF][START_REF] Zhang | Experimental Demonstration of Topological Surface States Protected by Time-Reversal Symmetry[END_REF]. Recently, it has been reported that the band structure of TI can be engineered by fabricating alloys of Bi2Te3 and Sb2Te3 using molecular beam epitaxy (MBE) to form the corresponding ternary (Bi1-xSbx)2Te3 alloy [START_REF] Zhang | Band structure engineering in (Bi1-xSbx)2Te3 ternary topological insulators[END_REF][START_REF] Weyrich | Growth, characterization, and transport properties of ternary (Bi1-xSbx)2Te3 topological insulator layers[END_REF]. The topological surface state (TSS) and the associated energy position of the Dirac point (DP) in the band gap can then be tuned systematically with different Bi/Sb concentrations. The transport properties of ternary TI alloys are also consistent with that of a single spin-polarized Dirac cone. This allows to overcome the disadvantage of binary topological insulators Bi1-xSbx [START_REF] Hsieh | A topological Dirac insulator in a quantum spin Hall phase[END_REF], Bi2Se3 or Bi2Te3 [START_REF] Hsieh | Observation of Time-Reversal-Protected Single-Dirac-Cone Topological-Insulator States in Bi2Te3 and Sb2Te3[END_REF][START_REF] Xia | Observation of a large-gap topological-insulator class with a single Dirac cone on the surface[END_REF], which always show conductive bulk states due to the defect-induced charge carriers. Therefore, the tunable Dirac cone structure in the ternary (Bi1-xSbx)2Te3 TI allows to develop topological p-n junction devices [START_REF] Kim | Atomically Abrupt Topological p-n Junction[END_REF][START_REF] Tu | In-plane topological p-n junction in the three-dimensional topological insulator Bi2-xSbxTe3-ySey[END_REF].

Recently, the discovery of giant spin Hall angle at room temperature in ferromagnet/TI heterostructures has also attracted intensive interests due to the possible applications to advanced spintronic devices, such as spin-orbit-torque magneto-resistive random-access memories and spin-torque oscillators [ 12 , 13 , 14 , 15 ]. Experimentally, to fabricate metal/TI heterostructures, it is required to either (1) build sophisticated integrated multi-deposition systems to grow in-situ the TI and other materials (metals, oxides, semiconductors) without breaking the vacuum, or (2) deposit a suitable capping layer on TI before the transfer through air and then remove the capping layer in another UHV chamber. As reported, the protection by the Se capping layer is not efficient because of the replacement of Te by Se up to 7 nm deep region beneath the surface, which strongly changes the surface chemical composition and modifies the surface state after desorption [START_REF] Virwani | Controlled removal of amorphous Se capping layer from a topological insulator[END_REF]. Some works have reported the use of Te for the capping layer on binary Bi2Te3 [START_REF] Hoefer | Protective capping of topological surface states of intrinsically insulating Bi2Te3[END_REF][START_REF] Fornari | Preservation of pristine Bi2Te3 thin film topological insulator surface after ex situ mechanical removal of Te capping layer[END_REF], but the influence of Te on the stoichiometry and on the SSB of ternary (Bi1-xSbx)2Te3 after desorption is still unclear.

In this study, we have performed a systematic study of the surface morphology, crystallinity, chemical composition as well as the band structure of (Bi1-xSbx)2Te3 after the desorption of the Te capping layer. Our results confirm a good recovery of SSBs of the (Bi1-xSbx)2Te3 ternary topological insulators with different Bi and Sb compositions. We have found that the chemical composition of (Bi1-xSbx)2Te3 remains almost unchanged after the desorption of Te. This study proves that the Te capping layer works as a suitable protection for ternary (Bi1-xSbx)2Te3 TI layers, allowing for ex-situ transfer of TI samples in air.

Experimental methods

The (Bi1-xSbx)2Te3 ternary topological insulators were grown by co-evaporation method in a MBE system with a base pressure of 510 -10 torr. Firstly, the dielectric SrTiO3(111) substrates were annealed at 930°C in oxygen atmosphere for 3 hours. Ten quintuple layers (QLs) of (Bi1-xSbx)2Te3 layers were then grown at 250°C followed by an annealing procedure at 280°C for half an hour. The growth rate was calibrated by a real-time reflection high-energy electron diffraction (RHEED) intensity oscillation. Supplementary Fig. S1 shows the RHEED pattern taken on one sample of 10 QL thick (Bi0.19Sb0.81)2Te3. The sharpness of the feature provides a clear evidence for the high quality of the sample. Two different chemical composition of (Bi0.19Sb0.81)2Te3 and (Bi0.75Sb0.25)2Te3 were prepared in this study. The films were capped with Te layer at room temperature before being exposed to air. The RHEED pattern after deposition of Te capping layer (Supplementary Fig. S2) completely loses all diffraction lines and indicates an amorphous nature of the Te layer. The details of growth condition can be found elsewhere [START_REF] Zhang | Band structure engineering in (Bi1-xSbx)2Te3 ternary topological insulators[END_REF].

The sample was then transferred to another ultra-high vacuum (UHV) multi-chamber interconnected system, where we firstly performed the desorption of Te layer. The desorption procedure was controlled by X-ray photoemission spectroscopy (XPS). The sample surface crystallinity was characterized by RHEED and low-energy electron diffraction (LEED). The sample morphology was characterized by scanning electron microscopy (SEM). Scanning Auger microscopy (SAM) and Auger electron spectroscopy (AES) were used to examine the chemical distribution at the surface. Finally, angle resolved photoemission spectroscopy (ARPES) was employed to study the band structure.

Results and discussion

Control of Te desorption procedure with XPS

The Te desorption process was controlled by XPS measurements. XPS spectra have been obtained using a monochromatized Al-K X-rays source (1486.6eV) with an energy resolution of 250 meV. Our previous experiments show that the stoichiometry of the sample was changed in an irremediable manner when it was heated up to 240°C. To avoid damaging the sample structure, we have carefully heated the sample to 200°C with a ramp of 10°C/min. XPS spectra were recorded each one hour after cooling down the sample from 200°C to the room temperature. Since the STO substrate is insulating, a large charging effect produces a shift of about 40eV in the XPS spectra observed. To remove this charging effect, we have deposited 10nm of Au at the sample edge when the Te desorption procedure was completed. As identified in Fig. 1(c), the Fermi level of the sample revealed from the rising of density of states of valence band (VB) is exactly located at zero binding energy (BE), indicating no charging effect after deposition of Au. to estimate the thickness of the Te capping layer according to the formula: 𝐼 = 𝐼 𝑒𝑥𝑝 / [START_REF] Hirokawa | Estimation of the thickness or composition of a covering layer on a solid by XPS or AES[END_REF], where I and I0 are the intensity of peak before and after Te desorption, respectively. t is the thickness of Te capping layer and  is the inelastic mean free path (IMFP) at the BE of elemental peak [START_REF] Seah | Quantitative Electron Spectroscopy of Surfaces: A Standard Data Base for Electron Inelastic Mean Free Paths in Solids[END_REF]. By using the Bi 4f7/2 core level (2nm) for the estimation, we can estimate the thickness of the Te capping layer to be about 3nm. Fig. 2(c-e) show the Te 3d, Bi 4f and Sb 3d core level spectra for the two samples with different chemical compositions ((Bi0.19Sb0.81)2Te3 and (Bi0.75Sb0.25)2Te3) after desorption. Both Te 3d and Bi 4f core levels exhibit no shoulder, indicating that there is no more surface oxidation layer. However, small peaks around 529eV and 533eV can be still distinguished beside the metallic contribution of Sb 3d5/2. Since no similar peaks are observed beside the contribution of Sb 3d3/2, we can then exclude the possibility from the oxidation of Sb.

Therefore, these small peaks could be attributed to the O1s core level resulting from the contamination on the sample holder which is also probed by X-rays due to the large illumination area, as mentioned before. By comparison of the two samples, the tendency of the intensity change in Bi 4f and Sb 3d peaks agrees with the composition of samples. However, the intensity change is still much smaller than what we expect from the nominal growth concentration. In fact, we expect a 4 and 3.5 times change in Bi and Sb signals, respectively. However, the change of Bi 4f and Sb 3d intensities are only 1.3 and 2 times, respectively. In order to determine the surface stoichiometry, we have acquired spectra of Te 4d, Sb 4d and Bi 5d of the (Bi0.19Sb0.81)2Te3 sample, as shown in Fig. 1(b). Since these peaks are located in a narrow BE range of 20-40eV, we can avoid the different IMFP for different elements. After subtraction of the background, the Te 4d, Sb 4d and Bi 5d spectra have been fitted to extract the integrated area under the peaks. By taking into account the atomic sensitivity factor: Te 4d: 0.97, Sb 4d: 0.86, Bi 5d: 0.65 (from Ref. [START_REF] Wagner | Empirical atomic sensitivity factors for quantitative analysis by electron spectroscopy for chemical analysis[END_REF]), we obtain the stoichiometry to be about Bi:Sb:Te=1.5:0.8:3. This composition is different than the nominal composition of Bi:Sb:Te=0.4:1.6:3 determined by the flux ratio Sb/Bi during the growth [START_REF] Zhang | Band structure engineering in (Bi1-xSbx)2Te3 ternary topological insulators[END_REF]. Since XPS results are more surface sensitive, our results indicate that the surface stoichiometry of TI could be different than the bulk one.

Study of surface crystallinity, morphology and chemical composition after Te desorption RHEED and LEED have been used to characterize the surface crystallinity after desorption. In the inset of Fig. 3(a), the RHEED pattern is taken with an electron energy of 15keV at grazing incidence, i.e. parallel to the sample surface. Before Te desorption, RHEED pattern shows a featureless diffusive pattern due to the amorphous character of Te capping layer (Supplementary Fig. S1(b)). After Te desorption, the sharpness of the feature is compatible to the pattern before Te deposition (Supplementary Fig. S1(a)), which proves the high quality of the sample surface. The inset of Fig. 3(b) displays the LEED pattern taken with an electron energy of 59eV at normal incidence, i.e. perpendicular to the sample surface. In addition to the six-fold symmetric spots related to the diffraction of the epitaxially grown (111) plane of (Bi0.19Sb0.81)2Te3, a ring associated to a polycrystalline structure is also observed. The different information given by RHEED and LEED could be related to their specific geometry and energy of electrons (coherence length) for characterization, which allows us to get complementary information on the surface crystallinity of the sample. Generally, LEED is more reliable for the analysis of defect structure due to the low inelastic scattering with low incident electron energy [START_REF] Mfiller | Comparison of reflection high-energy electron diffraction and low energy electron diffraction using high-resolution instrumentation[END_REF].

To understand the origin of the polycrystalline ring, the surface morphology is examined by SEM. Fig. 3(a) and (b) show the SEM images with different scales. In the large-scale image in Fig. 3(a), many flake-like island structures with size of several microns can be observed on the relative flat surface. These islands have random shape but all exhibit 2D growth mode with flat surface. In the magnified image in Fig. 3(b), some protrusion structures with small size of 50nm can be distinguished. These protrusion structures are homogeneously distributed on the whole surface. To check if the flake-like islands and the protrusion features are related to the procedure of Te desorption, we have measured the surface morphology of one pristine TI surface before Te deposition with atomic force microscopy (AFM). The sample was scanned by AFM immediately after being taken out from UHV growth chamber. In Supplementary Fig. S2(b), similar flake-like feature can be distinguished with a much smaller island size (about 100nm) which could be due to the different chemical concentration of the observed sample. In the large-scale AFM image (Supplementary Fig. S2(a)), the protrusion structures with the same density and size can be found in the pristine surface. These results confirm that the specific features observed after Te desorption already exist on the TI pristine surface and the Te desorption procedure does not modify the surface morphology. The islands could be related to the 2D growth mode of (Bi0.19Sb0.81)2Te3, which are firstly grown laterally and coalesce with each other to form the full film.

Harrison et al. have reported the growth of Bi2Te3 thin films on Al2O3(0001) substrate by MBE [START_REF] Harrison | Two-step growth of high quality Bi2Te3 thin films on Al2O3 (0001) by molecular beam epitaxy[END_REF]. Similar protrusion 3D defect structures have been found on the surface of Bi2Te3, which are attributed to the limited temperature window for Bi2Te3 thin film growth. In our case, the protrusion structures could also be related to the 3D defect structure with small mis-orientated (Bi0.19Sb0.81)2Te3 grains. However, at present stage, we cannot conclude that the polycrystalline ring found in LEED pattern is due to the protrusion 3D defects or the flakelike islands. To definitively answer this question, further spatial resolved surface structure analysis should be performed.

To characterize the distribution of surface chemical composition, we have performed scanning Auger microscopy (SAM) with a spatial resolution of 27nm. Fig. 3(e) shows the typical AES spectra for energy ranges of 88-109eV and 420-500eV on the island part, where we can distinguish the peaks of Bi, Sb and Te located at kinetic energy (KE) of 97eV, 450eV and 479eV, respectively. To explore the chemical distribution at the surface, we have chosen one zone including the island structure, as shown in the SEM image in Fig. 3(c).

Since Sb and Te are very suitable for comparison due to the very closed KE and related IMFP, we have performed the mappings of Sb and Te elements, as shown in Fig. 3(d) and (f), respectively. In both images, we can observe a clear intensity contrast which well corresponds to the morphology of the surface in the vicinity of the island. The origin of this contrast is found to be related to the topography of the surface but not to its chemical composition. This has been checked by estimating the difference of intensity ratio of ISb/(ITe+ISb) inside and outside the island, which is found to be less than 2%. Furthermore, we also found the same intensity ratio in the protrusion structure (see Supplementary Fig. S3). Therefore, we can conclude that the chemical composition is homogeneous on the (Bi0.19Sb0.81)2Te3 surface after Te desorption.

Band structure characterization with ARPES

We have studied the band structure of two samples with different concentrations (x=0.81 and x=0.25) by means of ARPES. ARPES measurements have been performed at a low photon energy using Xe-I radiation (h=8.44eV) for the x=0.81 sample and using He-I radiation (h=21.22eV) for the x=0.25 one with an overall energy resolution including UV sources and analyzer characteristics better than 10 meV. The ARPES signal is expected to be more bulk sensitive by using Xe-I than He-I. However, we found that a higher signal to noise ratio was obtained with the Xe-I UV lamp compared to the He-I one. We also measured constant energy maps by using the deflection mode of our Scienta-omicron DA30 L photoemission analyzer, without rotating the sample. Here, the kx axis corresponds to the standard measurement direction obtained along the analyzer slit, whereas the ky axis corresponds to the perpendicular direction by using the deflectors. Fig. 4(c) shows the ARPES spectrum obtained on the (Bi0.19Sb0.81)2Te3 sample after Te desorption. We can clearly observe the existence of the TSS within the gap of bulk states. The bulk valence band (BVB) is visible but not the bulk conduction band (BCB) associated with the n-type impurities. The TSS has a linear dispersion with two branches intersecting at DP at  point(i.e. at k// = 0). These two branches have an opposite spin texture. The BE of the DP is ED=-15020 meV. The Fermi wave vectors kF have been measured to be kF=0.06 Å -1 . The Fermi velocity VF of the linear dispersion of the Dirac cone following E-EF=ħVF(k-kF) is equal to 2.460.05×10 6 m/s. To compare with the band structure of the pristine TI sample without Te capping, we have adopted the measured band structures for (Bi1-xSbx)2Te3 (x=0.88 and 0.75) published from the same group [START_REF] Zhang | Band structure engineering in (Bi1-xSbx)2Te3 ternary topological insulators[END_REF], as shown in Fig. 4(a) and (b). Since the energy position of DP (ED) is very sensitive to the stoichiometry of TI sample, this allows us to examine if the stoichiometry is changed after Te desorption. Interestingly, our results highlight that the DP position at ED=-0.15 eV for (Bi0.19Sb0.81)2Te3 after Te desorption is well seated intermediately between the DP positions of (Bi0.12Sb0.88)2Te3 (ED=-0.12eV) and (Bi0.25Sb0.75)2Te3 (ED=-0.21eV), which is in perfect agreement with Ref. [START_REF] Zhang | Band structure engineering in (Bi1-xSbx)2Te3 ternary topological insulators[END_REF]. Therefore, we can conclude a high quality of TI surface despite the process of Te capping and desorption. Furthermore, our results prove the high performance of Xe lamp, especially its high photon flux to discern the TSS band with very low spectral weight and located in the vicinity of the  point. Fig. 4(d) and (e) present the constant energy maps at the Fermi energy (EF) and at the DP energy position (ED), respectively. In Fig. 4(d), the Fermi surface of the TI alloy displays an almost circular shape (black dashed circle), which is in agreement with an alloy with small concentration of Bi, unlike the case of Bi2Te3 [START_REF] Souma | Direct measurement of the out-of-plane spin texture in the dirac-cone surface state of a topological insulator[END_REF]. The circle diameter is shrunk for the energy close to DP (Fig. 4(e)), which validates the cone structure of TSS band.

The second TI sample (Bi0.25Sb0.75)2Te3 with different Bi/Sb ratio has also been characterized with ARPES after the same procedure of desorption of Te capping layer. The band structure has been recorded with a He I lamp source (h=21.2eV), as shown in Fig. 5(b). Again, we can clearly observe the TSS dispersion in the gap of the bulk states. For this sample, the signature of BCB can be observed at the Fermi level inside the Dirac cone. The BVB is very similar to what has been observed in the case of a pure Bi(111) sample [START_REF] Ohtsubo | Non-trivial surface-band dispersion on Bi(111)[END_REF]. The BE of the DP is ED=-25020 meV. The Fermi wave vectors have been measured to be kF=0.12 Å -1 , close to the characteristic of compound Bi2Te3 [START_REF] Souma | Direct measurement of the out-of-plane spin texture in the dirac-cone surface state of a topological insulator[END_REF]. The Fermi velocity of the linear dispersion of the Dirac cone following E-EF=ħVF(k-kF) is equal to 1.000.05×10 7 m/s. We have also compared our results with the published result measured with the pristine TI sample with the same concentration [START_REF] Zhang | Band structure engineering in (Bi1-xSbx)2Te3 ternary topological insulators[END_REF] (Fig. 5(a)). The two results possess almost the same features, which validates the fact that the TSS is unchanged after Te desorption. In Fig. 5(c), we have plotted a series of momentum distribution curves (MDC) at E=-45meV (close to EF), -115meV and -230meV (close to ED). It allows us to better disentangle the band structure of the system. On one hand, MDC at E=-45meV and -115meV can be well fitted with three components which are related to the TSS and BCB dispersions. On the other hand, only one peak centered at kx=0 is found at E=ED. It corresponds to the crossing point of the two TSS branches at the  point. Fig. 5(d-g) display the constant energy maps recorded at different energies. Since the stoichiometry of the second TI sample is different from that of the first sample we studied, a change in the topology of the Fermi surface is expected. This is indeed what we observe in Fig. 5(e) at E=EF-40meV. We can see the electron pocket at the center of the Brillouin zone (kx=ky=0) corresponding to the BCB, as well as the deformation of the Fermi surface associated to the TSS. The singular topology "snow flake" of the Fermi surface of the spinpolarized TSS (marked with black dot line) can be well understood like in the case of Bi2Te3 material [START_REF] Souma | Direct measurement of the out-of-plane spin texture in the dirac-cone surface state of a topological insulator[END_REF], which is also in good agreement with the data of the literature [START_REF] Nomura | Relationship between Fermi surface warping and out-of-plane spin polarization in topological insulators: A view from spin-and angle-resolved photoemission[END_REF]. At E=EF-100 meV (Fig. 4(f)), the isoenergetic surface regains a uniform circle character. Finally, at lower BE (E=EF-250 meV) (Fig. 4(g)), the isoenergetic surface of the valence band is characteristic of a Bi(111) sample with this six-branched "flower" form [START_REF] Ohtsubo | Non-trivial surface-band dispersion on Bi(111)[END_REF], confirming the Bi rich composition in the measured (Bi0.75Sb0.25)2Te3 sample.

Fig. 1 (

 1 a) shows the large-scale XPS spectrum of the (Bi0.19Sb0.81)2Te3 sample after desorption of Te. Besides of Te, Sb and Bi signals, peaks related to C, Mo, O and Au elements can also be observed. These extra signatures originate from the sample holder and from the edge of the sample recovered by Au due to the large illumination area of X-rays source and the large entrance of the analyzer.

Fig. 2 (

 2 Fig.2(a) shows Te 3d spectra before and after desorption. Before Te desorption, small shoulders appear

FiguresFigure 1 :

 1 Figures

  Figure 1: (a) General XPS spectrum obtained with monochromatized Al-K radiation (1486.6 eV) on the

Figure 2 :

 2 Figure 2: (a) Zoom on the Te 3d core level before the desorption (black curve) and after the desorption (red

Figure 3 :

 3 Figure 3: (a,b) SEM images with different scales for (Bi0.19Sb0.81)2Te3 sample after desorption. Inset of (a):

Figure 4 :

 4 Figure 4 : (a,b) ARPES intensity maps on pristine (Bi1-xSbx)2Te3 surfaces without Te capping obtained from

Figure 5 :

 5 Figure 5 : (a) ARPES intensity maps on pristine (Bi1-xSbx)2Te3 surfaces without Te capping obtained from

Acknowledgement Y.Lu acknowledges the support from the French National Research Agency (ANR) FEOrgSpin project (Grants No. ANR-18-CE24-0017-01) and ICEEL international SHATIPN project. K. Zhu and K. He acknowledge the support from the National Key Research and Development Program of China under Grant No 2017YFA0303303. Experiments were performed using equipment from the platform TUBE-DAUM funded by FEDER (EU), ANR, the Region Lorraine and Grand Nancy.

Summary

We have performed a systematic study of the surface morphology, crystallinity, chemical composition as well as the band structure of (Bi1-xSbx)2Te3 after the desorption of Te capping layer. Our results confirm a good recovery of SSBs of the (Bi1-xSbx)2Te3 ternary topological insulators with different Bi and Sb compositions.

The chemical composition of (Bi1-xSbx)2Te3 remains almost unchanged after the desorption of Te. This study proves that the Te capping layer works as a suitable protection for ternary TI layers, allowing for ex-situ transfer of TI samples in air. This opens the way for the development of metal/TI hybrid structures for advanced spintronic applications.