Erse Jia

Chen Xie
email: xie_chen@tju.edu.cn

Na Xiao

Francois Courvoisier

Minglie Hu

Two-photon polymerization of femtosecond high-order Bessel beams with aberration correction

Keywords: Femtosecond laser, Two-photon polymerization, Aberration correction, Bessel beams

In the femtosecond two-photon polymerization (2PP) experimental system, optical aberrations degrade the fabrication quality. To solve this issue, a multichannel interferometric wave-front sensing technique is adopted in the adaptive laser processing system with a single phaseonly spatial light modulator. 2PP fabrications using corrected high-order Bessel beams with the above solution have been conducted and high-quality microstructure arrays of micro-tubes with 20 microns diameter are rapidly manufactured. The effectiveness of the proposed scheme is demonstrated by comparing the beam intensity distributions and 2PP results before and after aberration corrections.

Introduction

Femtosecond two-photon polymerization [START_REF] Kawata | Finer features for functional microdevices[END_REF][START_REF] Jiang | Performance comparison of acrylic and thiol-acrylic resins in two-photon polymerization[END_REF][START_REF] Chu | Centimeter-Height 3D Printing with Femtosecond Laser Two-Photon Polymerization[END_REF][START_REF] Li | Achieving λ /20 Resolution by One-Color Initiation and Deactivation of Polymerization[END_REF][START_REF] Frenzel | Three-dimensional mechanical metamaterials with a twist[END_REF] (2PP) is widely used in the fabrication of three-dimensional complex architectures with a sub-micrometer resolution. Recent applications are templates for quantum dots [START_REF] Bogucki | Ultra-long-working-distance spectroscopy of single nanostructures with aspherical solid immersion microlenses[END_REF], scanning-probe microscopes [START_REF] Dietrich | 3D -Printed Scanning -Probe Microscopes with Integrated Optical Actuation and Read-Out[END_REF], microchips [START_REF] Wang | Multilayered skyscraper microchips fabricated by hybrid "all-in-one" femtosecond laser processing[END_REF], and biomimetic 4D printing [START_REF] Ma | Femtosecond laser programmed artificial musculoskeletal systems[END_REF]. The conventional 2PP procedures utilize the tightly-focused femtosecond laser beam to trigger polymerization inside the photoresist. In recent years, femtosecond 2PP based on adaptive optics [START_REF] Salter | Adaptive optics in laser processing[END_REF][START_REF] Gittard | Fabrication of microscale medical devices by two-photon polymerization with multiple foci via a spatial light modulator[END_REF][START_REF] Li | Multifocal optical nanoscopy for big data recording at 30 TB capacity and gigabits/second data rate[END_REF][START_REF] Salter | Dynamic control of directional asymmetry observed in ultrafast laser direct writing[END_REF] has attracted considerable interest due to the enhanced fabrication efficiency with different structured light beams, such as Bessel beams [START_REF] Yang | Two-photon polymerization of cylinder microstructures by femtosecond Bessel beams[END_REF], optical vortex beams [START_REF] Mills | Single-pulse multiphoton fabrication of high aspect ratio structures with sub-micron features using vortex beams[END_REF][START_REF] Stankevicius | Fabrication of micro-tube arrays in photopolymer SZ2080 by using three different methods of a direct laser polymerization technique[END_REF][START_REF] Yang | Direct laser writing of complex microtubes using femtosecond vortex beams[END_REF], abruptly autofocusing beams [START_REF] Manousidaki | Abruptly autofocusing beams enable advanced multiscale photo-polymerization[END_REF] and axilens beams with long focal depths [START_REF] Pan | Efficient fabrication of a high-aspect-ratio AFM tip by one-step exposure of a long focal depth holographic femtosecond axilens beam[END_REF]. Within these procedures, the spatial light modulators (SLM) are introduced to generate arbitrary optical patterns through computer generated hologram (CGH) algorithm [START_REF] Yang | Rapid Two-Photon Polymerization of an Arbitrary 3D Microstructure with 3D Focal Field Engineering[END_REF] so that various structures could be fabricated via one-step exposure. In fact, many factors in the 2PP setup combining the ultrafast laser and the adaptive optics can introduce undesired aberrations, such as the walk-off effect in the nonlinear crystal, the internal stress and the nonlinear response of the adaptive devices, and the manufacturing defects or assembly errors of the optical element.

On the other side, Bessel beams are notable for the diffractionresistance and self-healing behaviors. Besides these properties, high-order Bessel beams also show potentials to synthesize more complex structured lights [START_REF] Kontenis | Optical Drills by Dynamic High-Order Bessel Beam Mixing[END_REF][START_REF] Anand | Three-Dimensional Incoherent Imaging Using Spiral Rotating Point Spread Functions Created by Double-Helix Beams [Invited[END_REF]. Based on the above characteristics, high-order Bessel beams have been widely used in micro-nano manufacturing [START_REF] Wetzel | Femtosecond laser fabrication of micro and nano-disks in single layer graphene using vortex Bessel beams[END_REF], biological imaging [START_REF] Yu | Super-resolution deep imaging with hollow Bessel beam STED microscopy[END_REF], optical tweezers [START_REF] Volke-Sepulveda | Orbital angular momentum of a highorder Bessel light beam[END_REF], optical communication [START_REF] Zhang | Reducing orbital angular momentum crosstalk of the Bessel-Gaussian beam for underwater optical communications[END_REF][START_REF] Yuan | Beam wander relieved orbital angular momentum communication in turbulent atmosphere using Bessel beams[END_REF] and other fields. However, the aberrations in the setup significantly affect the application of high-order Bessel beams since their patterns are extremely sensitive to aberrations [START_REF] Miao | Phase front retrieval and correction of Bessel beams[END_REF].

Numerous methods can assist in shaping beams and/or removing aberrations in the optical system [START_REF] Wulff | Aberration correction in holographic optical tweezers[END_REF][START_REF] Jesacher | Wavefront correction of spatial light modulators using an optical vortex image[END_REF][START_REF] Liang | Aberration correction in holographic optical tweezers using a high-order optical vortex[END_REF][START_REF] Armstrong | Improved two-photon photopolymerisation and optical trapping with aberration-corrected structured light[END_REF][START_REF] Sanner | Programmable focal spot shaping of amplified femtosecond laser pulses[END_REF][START_REF] Mauclair | Ultrafast laser writing of homogeneous longitudinal waveguides in glasses using dynamic wavefront correction[END_REF], and the SLMs are the critical devices in these adaptive methods. Crucially, these solutions all require relatively advanced mathematical tools, such as the Zernike polynomial, the Gerchberg-Saxton or more complex algorithms. In the application of waveguide writing, the SLM has shown great power in correcting the spherical aberration inside bulky glass [START_REF] Mauclair | Ultrafast laser writing of homogeneous longitudinal waveguides in glasses using dynamic wavefront correction[END_REF] and controlling the nonlinear propagation [START_REF] Mauclair | Ultrafast laser writing of homogeneous longitudinal waveguides in glasses using dynamic wavefront correction[END_REF][START_REF] Xie | Tubular filamentation for laser material processing[END_REF].

Despite of the negligible nonlinear propagating effect in the application of 2PP, the SLMs are also playing critical roles by building fabricating systems based on adaptive optics. In such systems, a relay optics system is installed after the SLM to generate micron-scaled structured beams. Further filterings are usually introduced to eliminate the undesired diffracting subbeams resulting from the superimposed grating phases. Therefore, considering the increased complexity of these systems caused by the pixelated SLM, the above aberration correction approaches will no longer be applicable.

In this paper, we demonstrate the power of aberration correction in our 2PP experimental system based on the multichannel interferometric wave-front sensing technique. This method was originally developed to restore the intensity distribution of the structured beams in situ [START_REF] Čižmár | In situ wavefront correction and its application to micromanipulation[END_REF] requiring no complex mathematical tools. A single phase-only SLM is sufficient to implement this method into optimizing the 2PP system, sparing the heavy work of alignments in installing the system. We verify the effectiveness of this method by comparing the intensity profiles of the high-order Bessel beam before and after the correction. The experimental results of our aberration-free structured beam present excellent agreement with the simulations in the ideal case. With the help of this method, we finally realize the rapid fabrication of high-quality micro-tube structures by directly exposing the high-order Bessel beams into the negative photoresist.

Principle of the Aberration Correction

Any given optical field propagating through an optical system can be expressed as a composition of modes in any orthogonal representation [START_REF] Čižmár | In situ wavefront correction and its application to micromanipulation[END_REF]. The optimal focusing is achieved if and only if all the modes present the identical phase, which means the strength of constructive interference would be highest between these modes. Therefore, the optimal focusing generally requires accurately compensating the phase of each mode for any beam in the physical setup with imperfect optics. As an adaptive programmable device, the liquid crystal-based SLM can perform versatile tasks, including structured lights generation and optical field detection and/or correction. In our setup, we divide the twodimensional pixel array on the SLM into M*N channels with equal size. Each channel is filled with blazed grating phase with independent on-off switch, as shown in Fig. 1(a) and (b). With the blazed grating phase, the modulating information in each channel is coded in the first-order diffracting light and spatially separated from the undesired zero-order light. In the procedure of the wavefront sensing, we firstly select a channel as the phase reference and keep this channel on. Then, all the other test channels are successively switched on along the direction shown in Fig. 1(b). The intensity signal in the focal plane of the lens are collected while increasing the phase of the tested channel from 0 to 2π. For the convenience, the amplitude and phase of the i-th test channel and reference channel are respectively denoted by |Ui|, φi and |Ur|, φr. The interference intensity I of the first order of diffraction would follow the relation:

22 i r i r i r + 2 cos( ) I U U U U   + - (1)
Therefore, the phase of the test channel φi will be identical with the reference φr when the signal tends to reach the maximum. Note that the phase terms in Eq.( 1) represent the phase profiles in each channel, which is assumed as nearly homogeneous across the corresponding sub-regional wavefront. After probing all the test channels, the full wave-front would be plotted and a complementary phase map would be readily loaded on the SLM [Fig. 3. Experiments and Result 3.1. Setup The above aberration detection scheme could be readily integrated in our setup for 2PP as shown in Fig. 2. The laser source is a homemade femtosecond fiber laser amplifying system with standard double-cladding fibers similar to that in [START_REF] Song | Practical 24-fs, 1-μJ, 1-MHz Yb-fiber laser amplification system[END_REF], enabling 90-fs transform-limited laser pulses centered at 1038 nm output at 1-MHz repetition rate. A BBO crystal is inserted to generate the second harmonic (SH) wave for the 2PP fabricating process.

Considering the damage threshold of the coating on the crystal surface, the fabrication does not require the femtosecond laser to work at full load. After expansion, the SH wave of a diameter ∼ 8 mm illuminates the phase-only SLM (Holoeye PLUTO, 1920x1080 pixels) with the exposed power adjusted by the neutral density (ND) filter. The telescope composed by the lens (f=1m) and the microscope objective (Olympus 20×, NA=0.4) shrinks the modulated waves into the micro-scaled patterns. A mechanical shutter is used to control the exposure time in the sample. The CCD camera is located at the focal plane of the lens to perform the wave-front detection. In order to achieve more accurate wave-front detection, the number of divided areas on the SLM is in principle the more the better. However, a single channel with very few pixels would be too problematic to implement, resulting in too weak intensity of the interference patterns to be detected. Since the optical aberration in a typical device is a smooth function with low spatial frequencies, dividing the pixel panel into no less than 10×10 channels can provide sufficient sampling. To ensure the fineness of the detected wave-front, we divided the SLM panel into 20×20 channels in our tests together with π/10 phase increment. After probing the whole panel with the scheme in the previous section, we obtain the compensating phase as shown in Fig. 1(c). We note here that no great effort is made in aligning the beam path, leading to a significant aberration introduced in terms of misalignment. It is noteworthy that more than 4π phase fluctuation across the beam are detected, and thus a highly distorted field from the desired pattern would be expected. We remark that a high-quality objective from Olympus is adopted in the current setup to suppress the aberrations after the long focal lens in the telescope. But it is also possible to correct the aberration of the whole optical system with our scheme by inserting another lens after the objective and monitoring the signal in its focal plane. This is not realized in the current setup due to the rather limit space. Further engineering the setup is also part of our future work.

Aberration correction

As in [START_REF] Xie | Light trajectory in Bessel-Gauss vortex beams[END_REF], we apply the phase below on the input beam to generate high-order Bessel beams:

( , ) sin( ) r k r m     = - + (2)
where k is the wave vector in vacuum, γ is the cone angle between the refracted conical wave and the optical axis and m is the vortex order. In this paper, we take m=6 and γ=4°. Note that a blazed grating phase was also added to the SLM to separate the desired beam from the unmodulated zero-order light [START_REF] Zhang | Elimination of a zeroorder beam induced by a pixelated spatial light modulator for holographic projection[END_REF]. The Fourier spectra of the Bessel beams are further recorded, showing the distorted profile without correction in Fig. 1(d). After superimposing the compensating mask, the Fourier spectra is well corrected in terms of both circularity and intensity uniformity [Fig. 1(e)]. Besides, a slightly shrunk radius also occurs after the correction, which can be explained by the overall converging property of the compensating phase map. Fig. 3 presents the longitudinal and transverse intensity profiles of our high-order Bessel beams in direct space after the objective MO in Fig. 2. Numeric results of the Bessel beams generated from an ideal Gaussian beam are presented in the first row of Fig. 3 for comparison. The simulations are performed based on angular spectrum method as in our previous studies [START_REF] Xie | Light trajectory in Bessel-Gauss vortex beams[END_REF][START_REF] Xiao | Caustic Interpretation of the Abruptly Autofocusing Vortex beams[END_REF]. The second and third rows show the corresponding intensity profiles in experiments before and after the correction, respectively. After the transitionally expanding region along propagation [START_REF] Xie | Light trajectory in Bessel-Gauss vortex beams[END_REF][START_REF] Xiao | Caustic Interpretation of the Abruptly Autofocusing Vortex beams[END_REF], the main lobe converges to a constant diameter (~20μm) in the nondiffracting regime as shown in Fig. 3(g). However, the uncorrected main lobe cannot maintain its non-diffracting feature as shown in Fig. 3(b). Meanwhile, the annular intensity profile has also been seriously distorted with less annular rings. As expected, the correction operation has significantly reduced the detrimental aberrations, as shown in Fig. 3(c) and (f). The shape of the central intensity tube returns to the ideal trajectory and the intensity profile also restore its original pattern. Overall, the high-order Bessel beams after aberration correction are highly consistent with the ideal beam in the simulation.

Results and analyses

Encouraged by the high consistency between the restored and the ideal beam profile, we perform the two-photon polymerization as a further test. In the 2PP fabrication, the negative photoresist (MicroChem SU-8 2075, diluted with cyclopentanone, with the ratio of SU-8 : cyclopentanone = 5 : 1) is uniformly spin-coated on the glass slide substrate at 1500rpm. Soft baking was performed on a hot plate for 5 min at 65 °C and then for 10 min at 95 °C. Up to 1.5 mW average power of the ultrafast SH wave is exposed in the prepared sample at 1-MHz repetition rate, corresponding to 1.5nJ single pulse energy. The relatively low repetition rate can well avoid the occurrence of thermal issue in the manufacturing process. Considering the transitionally expanding ring lobe along propagation may occur more significantly for Bessel beams of certain parameters as shown in Fig. 3 and discussed in our previous work [START_REF] Xie | Light trajectory in Bessel-Gauss vortex beams[END_REF][START_REF] Xiao | Caustic Interpretation of the Abruptly Autofocusing Vortex beams[END_REF], non-diffracting region should be carefully positioned to cross the whole photoresist layer to guarantee the fabrication of well-shaped micro-tubes as shown in the inset of Fig. 2. Once the axial position of the beam is well selected, transversal movement in the horizontal plane together with exposure at proper power level would allow the rapid fabrication of the micro-tube array. The average time to fabricate each single micro-tube, including the movement and the exposure, is no more than 0.8sec without optimization. After the hard baking following the exposure, the photoresist is developed to produce the micropatterns as high as 30μm as shown in Fig. 4. The white dashed lines with arrows in Fig. 4(c) and (f) outline the outer walls of the micro-tubes before and after the aberration correction. It is noteworthy that the aberration also leads to the tapered wall in Fig. 4(c), i.e. a transitional polymerization region between the glass substrate and the micro-tube bottom. Most importantly, the non-uniform intensity of the uncorrected main lobe would introduce a much higher fluctuation of the local intensity during the 2PP process, which further degrades the inconsistency of the micro-tube wall thickness. Besides, we also found that the micro-tubes with non-uniform shaped walls have higher rates to fall down on the glass substrate. We attribute this to the structural shrinkage induced by the nonuniform light energy deposition along the distorted Bessel beams as shown in Fig. 3 [START_REF] Ovsianikov | Shrinkage of microstructures produced by two-photon polymerization of Zr-based hybrid photosensitive materials[END_REF]. After the phase correction, the above issues are well addressed. Once the high-order Bessel beams are well positioned across the interface between the substrate and the photoresist, no falling-down of the micro-tube structures are observed. In order to quantify the improvement in fabrication quality after the aberration removal, we extract the contours of the inner and outer rings of the microstructures in the SEM photos and measure their circularity. The ring contours are extracted by a combination of conventional operations in digital image processing, such as binarization, smoothing, filling and edge detection. The definition of circularity adopted here is: C=4π×A/P 2 , where A and P represent the area and perimeter of the selected contour, respectively. Obviously, the C value closer to unity indicates that the geometrical shape of the corresponding contour is closer to an ideal circle. Fig. 5 presents the circularity characterizations of the micro-tubes with identifying numbers in Fig. 4 (a) and (d). The average circularities of the outer rings before and after the correction are 0.919 and 0.893, accompanying with those of the corresponding inner rings, 0.956 and 0.939, respectively. It is worth noting that compared with the inner rings, the outer rings are more sensitive to the aberration in our setup. It is widely known that the vortex charge m can also affect the size of the main intensity lobe. To demonstrate validity of this approach, we also fabricate micro-tubes with Bessel beams of different vortex charges. In the fabrication, the laser power and the exposure time are adapted accordingly due to the change of the lobe size. Fig. 6 presents the SEM images of micro-tubes fabricated by high-order Bessel beams with vortex charges of 4, 6, and 8, and the measured outer rings diameters are 17.6 μm, 20.6 μm and 26.06 μm, respectively. All of the micro-tube structures present high-quality roundness and robust attachment to the substrate.

Conclusion

In this paper, we have demonstrated the powerful feasibility of the multi-channel interference wave-front sensing in adaptive-opticsbased two-photon polymerization. To demonstrate this, significant aberrations are intentionally introduced in the installation of our homebuilt system for ultrafast laser processing without great efforts. Without additional wave-front sensor, more than 4π wavefront fluctuation across the input beam can be well flattened with the integration of the above wave-front sensing scheme. This results in significant improvement of fabrication quality with corrected high-order Bessel beams. Since the integration of this wave-front sensing technique requires no knowledge on the advanced mathematical skills for workers, such as Zernikepolynomial-based decomposition, Gerchberg-Saxton iteration and/or more complex algorithms, this straightforward solution would be highly attractive to implement in the laser processing system based on adaptive optics. Besides, our work also shows a promising perspective on significantly lowering the cost of the fabricating system by sparing the ultrafast lasers with super highquality beams and exhausting alignments. We envision that our work will be very attractive in a broader scope of laser processing, where a well-defined wave-front is desired, such as the femtosecond laser-induced periodic surface structures [START_REF] Geng | Controllable generation of large-scale highly regular gratings on Si films[END_REF]. For present, the wave-front correction is performed just before the fabrication without any real-time feedback during the printing process. But we also notice that a novel monitoring method would allow real-time monitoring the printing [START_REF] Zvagelsky | Towards in-situ diagnostics of multi-photon 3D laser printing using optical coherence tomography[END_REF]. We remark that this method together with our technique would be a good solution to correcting not only the aberration from the optical system but also the distortions from other factors in the fabricating process. Future works may also be contributed to the fine feature correction with the complex amplitude modulation.

Fig. 1 .

 1 Fig. 1. Schematics of the wave-front sensing. (a) two channels on the SLM, CT: test channel, CR: reference channel; insets in (a) show the simulated interference patterns with various phase differences between the test and the reference channels: 0, π/2, π and 3π/2. (b) SLM area division and scanning path of the test channel; (c) the aberration correction mask loaded on the SLM; Fourier spectral profiles of the high-order Bessel beams (d) before and (e) after the correction.

Fig. 2 .

 2 Fig. 2. Schematic of the setup: BE, beam expander; HWP, halfwave plate; SLM, spatial light modulator; LB, laser block; ND filter, neutral density filter; BS, beam splitter; Shutter, mechanical shutter; MO, microscope objective; CCD, CCD camera. Inset close to the SLM presents the phase pattern to generate the Bessel vortex beams; the corresponding 3D light fields at the focus area in the slide are presented in the inset close to MO.

Fig. 3 .

 3 Fig. 3. Correction of the high-order Bessel beams with vortex charge m = 6: the longitudinal intensity profiles along propagation in (a) simulation and in experiments (b) before and (c) after the correction; corresponding transverse intensity profiles at z = 250μm in (d)-(f); (g) the evolution of the main lobe size along the propagation.

Fig. 4 .

 4 Fig. 4. SEM photos of micro-tube structures fabricated with highorder Bessel beams (a)-(b) before and (c)-(d) after aberration correction; the scaling bars without notation represent 20μm.The SEM images of the micro-tubes are obtained with ZEISS Gemini 300 electronic microscope as shown in Fig.4. For comparison, we firstly perform the fabrication of micro-tubes with the uncorrected high-order Bessel beams of vortex charge m=6. It can be seen in Fig.4(a)-(c) that the micro-tubes are seriously deformed due to the uncorrected Bessel beams in the nondiffraction region as shown in Fig.3. Fig.4 (d)-(f) present the microtube arrays with the single units fabricated by aberration-free high-order Bessel beams. It is apparent that the uniformity of the fabricated units is very satisfying. The measured outer ring diameters and wall thickness of the microstructures are ~ 20.8 μm and ~ 4.3 μm respectively.The white dashed lines with arrows in Fig.4(c) and (f) outline the outer walls of the micro-tubes before and after the aberration correction. It is noteworthy that the aberration also leads to the tapered wall in Fig.4(c), i.e. a transitional polymerization region between the glass substrate and the micro-tube bottom. Most importantly, the non-uniform intensity of the uncorrected main lobe would introduce a much higher fluctuation of the local intensity during the 2PP process, which further degrades the inconsistency of the micro-tube wall thickness. Besides, we also found that the micro-tubes with non-uniform shaped walls have higher rates to fall down on the glass substrate. We attribute this to the structural shrinkage induced by the nonuniform light energy deposition along the distorted Bessel beams as shown in Fig.3[START_REF] Ovsianikov | Shrinkage of microstructures produced by two-photon polymerization of Zr-based hybrid photosensitive materials[END_REF]. After the phase correction, the above issues are well addressed. Once the high-order Bessel beams are well positioned

Fig. 5 .

 5 Fig. 5. Circularity measurement results of the micro-tube structures: (a) the outer rings and (b) the inner rings.

Fig. 6 .

 6 Fig. 6. SEM photo of micro-tubes fabricated with high-order Bessel beams of different vortex charges.

Funding Sources. This work was supported by the National Natural Science Foundation of China (62275191, 61605142, 61827821); Tianjin Research Program of Application Foundation and Advanced Technology of China (17JCJQJC43500); Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences (Open Fund of the State Key Laboratory of High Field Laser Physics); European Research Council (682032-PULSAR); Agence Nationale de la Recherche (ANR-15-IDEX-0003, ANR-17-EURE-0002).


Aikaterini Kosntantinou
email: a.konstantinou@plymouth.ac.uk.

Timothy Scott

Gerd Masselink

Christopher Stokes

Daniel Conley

Bruno Castelle

SATELLITE-BASED SHORELINE DETECTION FOR MACRO-TIDAL COASTS: IMPACTS OF MORPHOLOGICAL AND HYDRODYNAMIC SETTING

Robust and accurate long-term mapping of the shoreline is a fundamental requirement for effective coastal management and policy making. Earth observation (EO) coupled with novel image analysis techniques have demonstrated the potential of EO for long-term, local, regional, and even global scale investigations of shoreline change. However, satellitederived shoreline (SDS) data is associated with large uncertainties relating to environmental factors, tidal range, and wave action among other factors. This contribution investigates the impacts of morphological and hydrodynamic setting on the accuracy of SDS in macrotidal, high-energy coasts. Results revealed significant differences in SDS accuracy between the two morphologically contrasting sites in terms of the level of error and the optimal water level correction. We show that SDS accuracy at macrotidal sites can be greatly improved by applying appropriate water level corrections and that a different approach is required depending on beach type (dissipative/reflective).

Introduction

Robust and accurate long-term mapping of the shoreline is a fundamental requirement for effective coastal management and policy making. However, changes along the shoreline occur over a range of timescales. On one end of the spectrum, intense storm events can result in extreme morphological change within hours on localized scales (e.g., [START_REF] Scott | Multi-annual sand and gravel beach response to storms in the southwest of England[END_REF][START_REF] Harley | Extreme coastal erosion enhanced by anomalous extratropical storm wave direction[END_REF], whilst at the other end of the spectrum, sea level rise drives coastal evolution at millennial timescales (e.g., [START_REF] Clemmensen | Morphology and sedimentary architecture of a beach-ridge system (Anholt, the Kattegat sea): a record of punctuated coastal progradation and sea-level change over the past similar to 1000 years[END_REF]. In between, decadal-scale cycles of shoreline change have often been reported, (e.g., [START_REF] Short | Decadal Scale Patterns in Beach Oscillation and Rotation Narrabeen Beach, Australia-Time Series, PCA and Wavelet Analysis[END_REF][START_REF] Turner | A multi-decade dataset of monthly beach profile surveys and inshore wave forcing at Narrabeen, Australia[END_REF]. Interannual and decadal timescales are of particular interest as they are closely linked to modes of global climate variability (e.g., [START_REF] Barnard | Coastal vulnerability across the Pacific dominated by El Nino/Southern Oscillation[END_REF].

In the North Atlantic, research indicates that the wave climate is strongly linked to large scale atmospheric controls. In particular, [START_REF] Castelle | A new climate index controlling winter wave activity along the Atlantic coast of Europe: The West Europe Pressure Anomaly[END_REF] showed that winter-averaged wave conditions in the region can be significantly explained by two climatic indices: the North Atlantic Oscillation (NAO) and the Western Europe Pressure Anomaly (WEPA). [START_REF] Scott | The extreme 2013/2014 winter storms: Beach recovery along the southwest coast of England[END_REF] noted a consistent decrease in winter-averaged easterly wave power around the Southwest coast of England since the 90's and hypothesized that regional wave forcing was linked to multiannual atmospheric variability. Recent research demonstrates that winter-averaged inshore wave conditions around the United Kingdom and Ireland, are linked to leading modes of climate variability in the North Atlantic, primarily the NAO, WEPA, and the Atlantic Oscillation (AO) [START_REF] Scott | Role of Atmospheric Indices in Describing Inshore Directional Wave Climate in the United Kingdom and Ireland[END_REF]. Research in the same region has also alluded to the possibility of regionally coherent patterns in coastal response to [START_REF] Burvingt | Classification of beach response to extreme storms[END_REF] and recovery from [START_REF] Konstantinou | The extreme 2013/14 winter storms: Regional patterns in multi-annual beach recovery[END_REF] extreme storm seasons. At the same time, a number of studies have attempted to link climate forcing patterns to changes in coastal landform [START_REF] Scott | The extreme 2013/2014 winter storms: Beach recovery along the southwest coast of England[END_REF][START_REF] Dodet | Beach recovery from extreme storm activity during the 2013-14 winter along the Atlantic coast of Europe[END_REF]. [START_REF] Burvingt | Climate forcing of regionally-coherent extreme storm impact and recovery on embayed beaches[END_REF], investigated the response of ten sites on the northern coast of SW England over a period of ten years (2007 to 2017) and founded strong connections between winter volume changes observed at the sites and winter averaged WEPA. Wiggins et al. (2019b), demonstrated that shoreline alignment in beaches at the south coast of England was largely explained by winter-averaged NAO and WEPA. Concomitantly, other studies have illustrated that NAO is also linked to beach state and nearshore bar dynamics through its control of winter wave conditions [START_REF] Masselink | Role of wave forcing, storms and NAO in outer bar dynamics on a high-energy, macro-tidal beach[END_REF]). Yet to date, our efforts to understand how sandy coasts in the North Atlantic respond to decadal cycles of basin-scale changes have been temporally and spatially limited to a small number of long-term (<16 years) beach monitoring sites.

Optical remote sensing can provide unique and powerful datasets of near global spatial coverage at moderate (10 to 30 m) spatial resolution and regular, short timescales (5 to 15 days), at low to no cost. Novel algorithms have demonstrated the potential of EO for long-term, local, regional, and even global scale investigations (e.g., [START_REF] Luijendijk | The state of the world's beaches[END_REF][START_REF] Bishop-Taylor | Mapping Australia's dynamic coastline at mean sea level using three decades of Landsat imagery[END_REF]. However, satellite coverage is not uniform across the globe and is further limited in certain areas due to high cloud cover and solar day length amongst other factors. Furthermore, satellite-derived shoreline data is associated with large uncertainties relating to various environmental parameters such as soil moisture, the presence of white water due to wave breaking, sediment type and grain size, and variability in the surrounding terrestrial zone. Tidal range and wave runup present additional challenges for satellite-based applications, particularly at low-gradient sites where small vertical changes in water level can translate to horizontal incursions of the shoreline in the order of hundreds of meters, especially when exposed to high-energy wave climates [START_REF] Castelle | Satellitederived shoreline detection at a high-energy meso-macrotidal beach[END_REF]. However, studies assessing the accuracy of satellitederived shorelines have thus far been largely restricted to microtidal, sandy beaches at lower latitudes. Quantifying and accounting for the effect of these factors on the accuracy of the derived shorelines will increase our confidence in applying EO data to describe shoreline evolution along high-energy, meso-macrotidal coasts. This contribution aims to examine the relationship between physical parameters such as still-water level, wave setup and runup on satellite-derived shorelines, and how this relationship varies with beach type in meso-to macrotidal settings with limited satellite data availability.

Methods

We use CoastSat (Vos et al., 2019b) to extract the shorelines and utilize a 14-year monthly morphological dataset consisting of RTK-GNNS measurements, combined with sea surface elevation and wave model hindcast data (Mercator Ocean 1.5-km reanalysis and UK Met Office 8-km WAVEWATCH III). We focus on two sites representing end members of beach morphological type: Perranporth (Fig. 1a,c) and Slapton Sands (Fig. 1b,d). We first aim to establish the measurement accuracy of the instantaneous SDS (SDW) and establish the optimum water level parametrization for each site. We test five different water level parametrizations: i) no consideration of water level fluctuations (η0); ii) including tidal elevation (ηt); iii) including tidal elevation and wave setup (ηts); iv) including tidal elevation and the full Stockdon wave runup (ηtsr*); and v) including tidal elevation and the full Stockdon wave runup after accounting for wave transformation (ηtsr).

We calculate wave set up and runup using the equations proposed by [START_REF] Stockdon | Empirical parameterization of setup, swash, and runup[END_REF]:

S 2% = 1.1 * 0.35 tan β H o L o (1) R 2% = S 2% + 1.1 * 0.5H o L o [0.563(tan β) 2 + 0.004] 0.5 (2)
where Lo, Ho are the deep-water wavelength and height, respectively, Hs is the significant wave height, Tp is the peak period, and tanβ is the beach slope. The deep-water wave parameters required in the Stockdon equation were obtained by reverse-shoaling the extracted model data [START_REF] Stockdon | Empirical parameterization of setup, swash, and runup[END_REF] from the location of the model output nodes to a depth of 1000 m using linear wave theory. To ensure that the derived deep-water wave conditions accounted for the effects of nearshore wave transformation, wave direction and height at breaking were calculated prior to reverse shoaling [START_REF] Stokes | Forecasting coastal overtopping at engineered and naturally defended coastlines[END_REF]. The breaking wave parameters were calculated using the approach proposed by van Rijn (2014) and Hunt's approximation of wavelength and wave celerity [START_REF] Hunt | Direction solution of wave dispersion equation[END_REF].

We then project the SDW to a relevant reference shoreline to get an equivalent shoreline (SDS) using different methods each requiring different levels of information on beach morphology: i) using a time-invariant transect profile; ii) using a beach-averaged, time-invariant beach slope; iii) using an estimated value for beach slope following [START_REF] Bujan | From fine sand to boulders: Examining the relationship between beachface slope and sediment size[END_REF].

Results

SDW accuracy

Fig. 3 shows the accuracy of the instantaneous satellite-derived waterlines (SDW) for each water level parametrization tested. Results indicate that including tidal elevation alone led to the highest SDW accuracy at Slapton (RMSE: 14.4 m; mean bias: 7.7 m, seaward) improving RMSE accuracy by 20% and the mean bias by 29%. At Perranporth, including tidal elevation also led to a significant reduction of the RMSE by 78%. However, when considering tidal elevation, the mean bias switched from 16.3 m seaward to 20.3 m landwards, illustrating the possible effect of tidal aliasing. At this site, considering wave-induced water-level fluctuations led to a further considerable reduction of the RMSE by 34% and the mean bias by 78% leading to an overall reduction of 85% in RMSE and 72% in the mean bias. The level of error associated with Perranporth (RMSE = 20.4 m; mean bias = 4.5 m, landward) remained significantly higher than that obtained at Slapton (RMS = 14.4 m; mean bias = 7.7 m, seaward. Interestingly there is a clear difference between the performance of Sentinel 2 (S2) and Landsat 8 (L8) from site to site. Fig. 3. Top row: SDW error (Wsat -Wη) at the two sites derived at every beach profile transect for each image/survey pair up to 30 days apart for the different water level parametrisations (η0; no correction; ηt: tidally corrected; ηts: tide and setup; ηtsr*: tide and full runup; ηtsr: tide and full runup after wave transformation). Bottom row: Satellite performance shown for the two sites based on the optimal configuration at each site (Slapton: using ηt; Perranporth: using ηtsr).

SDS accuracy

Further analysis only included the optimal parameterizations at each site based on the results obtain in the previous section, (i.e., ηt for Slapton and ηtsr for Perranporth). We examine several methods of translating horizontally a given waterline to some reference shoreline: (i) using a temporally averaged transect profile; (ii) using a mean slope calculated between HAT and LAT, similar to the approach adopted by Vos et al. (2019a) and [START_REF] Castelle | Satellitederived shoreline detection at a high-energy meso-macrotidal beach[END_REF] (Slapton: tanβ = 0.125; Perranporth: tanβ = 0.075). As both approaches require information on the shape of the beach profile, we also used empirical estimations of beach slope that are more relevant to sites for which no data is available. Consequently, we also translate the SDW to a reference elevation using iii) the empirical relationship proposed by [START_REF] Bujan | From fine sand to boulders: Examining the relationship between beachface slope and sediment size[END_REF] (Slapton: tanβ = 0.149; Perranporth: tanβ = 0.075); and iv) the 25 th and 75 th quartile beach slope values included in [START_REF] Bujan | From fine sand to boulders: Examining the relationship between beachface slope and sediment size[END_REF] for gravel and sandy beaches (Slapton: tanβ = 0.09 and 0.19 respectively; Perranporth: tanβ = 0.02 and 0.1 respectively). Fig. 4 shows a comparison of SDS RMSE and mean bias for Slapton (for η = ηt) and Perranporth (for η = ηtsr) obtained using these different methods to translate the cross-shore locations of the SDW (Wsat) to a range of reference shorelines between AMSL and AMSL + 3 m.

Discussion

This study addressed the influence of tidal elevation and wave-induced water level fluctuations on the accuracy of satellite-derived shoreline data at two different coastal environments in the UK. Results showed that uncertainty in SDS in macro-tidal beach settings can be significantly reduced by defining an appropriate water level parametrization . Further, our results highlighted marked differences in SDS accuracy between the two contrasting sites in terms of the level of error and the optimal water level correction. Results further revealed different, and spatially variable performance between satellite sensors. These findings suggest that the optimal strategy for accurate SDS measurement may be site-specific.

At both sites, accounting for tidal elevation led to a marked reduction in SDW RMSE of 20% at Slapton and 75% at Perranporth. At Perranporth, including the wave runup component was key to achieving a further significant accuracy improvement, reinforcing the findings of [START_REF] Castelle | Satellitederived shoreline detection at a high-energy meso-macrotidal beach[END_REF]. When this correction was applied to the reflective site (Slapton), however, it resulted in reduced accuracy, suggesting that the optimal SDW definition varies with beach type and should be considered in local-scale applications. Nevertheless, ηtsr was found to represent the optimal approach for applications that include a variety a different beach types.

In terms of SDS accuracy, the highest accuracy across the board was achieved by using an average measured profile for translating Wsat to a reference shoreline. For large-scale applications or when extracting shorelines at sites with little or no information, linear translation using some measure of beach slope is typically the only available option for translating Wsat to a reference shoreline. In this case, error was a function of both detection errors and the deviation of beach-face geometry from the theoretical line defined by the translation slope. Consequently, the level of error associated with this method was strikingly higher and greatly influenced the selected shoreline proxy at both sites (Fig. 4).

Conclusion

• Analysis found satellite-derived shoreline accuracy at selected UK sites was significantly improved by applying water level corrections.

• Results indicate a different correction approach is required for water level definition for different beach types (reflective, dissipative).

• Accuracy of SDS depends on chosen proxy shoreline and varies with beach type and shoreline translation method.

Fig. 1 .

 1 Fig. 1. Overview maps of Perranporth (a) and Slapton Sands (b) and locations of 1D survey profiles and 2D survey areas (Perranporth (c) Slapton (d)). Aerial photos of both sites show the transects along which the survey data was collected. The locations of the wave model nodes are also indicated.

Fig. 2 .

 2 Fig. 2. Wave climate and beach volume time series shown for the two sites. Significant wave height at Slapton (a) and Perranporth (b); alongshore wave power at Slapton (c) and overall wave power at Perranporth (d); beach volume time series for Slapton (e) and Perranporth (f).

Fig. 4 :

 4 Fig. 4: SDS accuracy achieved with different SDW translation methods at a range of reference shoreline elevations between AMSL and AMSL + 3.0m.

Acknowledgements

Our thanks go to all the CPRG members and University of Plymouth students who have been, and still are, contributing their efforts to the CPRG beach survey program. We would also like to thank Kilian Vos and the Water Research Laboratory for developing and making freely available the CoastSat toolkit and for their continuing support.


Kaili Zhang

Carole Rossi

Christophe Tenailleau

Veronique Conedera

CuO Nanowires Grown from Cu Film Heated Under a N 2 /O 2 Flow

Keywords: Pure, Aligned, CuO Nanowire, Copper Thin Film, Silicon, N 2 /O 2 Gas Flow

Pure, large-area, and aligned CuO nanowires are realized from copper thin film deposited onto silicon substrate by thermal annealing under a N 2 /O 2 gas flow. The as-prepared pure and vertically aligned CuO nanowires along silicon substrate have several advantages over previous approaches such as easier integration into silicon based micro systems, more convenience for subsequent elements deposition, and higher heat of reaction for realization of CuO/Al based nano energetic materials. It is found that long and aligned nanowires can only be formed within a narrow temperature range from 400 to 500 C. A N 2 /O 2 gas flow rate of 400/100 sccm is moderate for realizing long and aligned CuO nanowires. The synthesized nanowires and thin film beneath the nanowires are confirmed to be pure CuO using X-ray diffraction. The CuO nanowires are found to be bicrystal by high resolution transmission electron microscopy.

INTRODUCTION

Recently, nano copper oxides (mainly CuO and Cu 2 O) have become one intensive research focus because of their fascinating properties and interesting applications compared to their bulk or micro counterparts. Copper oxides have been extensively used in various areas including battery anode electrodes, heterogeneous catalyst, magnetic storage media, high-temperature superconductors, and nano energetic materials, [1][2][3][4][5][6] where one-dimensional nano copper oxides (wires, belts, and rods) have been paid special attention. One-dimensional nano copper oxides have been realized by different methods such as thermal decomposition of CuC 2 O 4 precursor, reduction of copper sulfate with hydrazine, heat treatment of Cu 2 S nanowires with oxygen, self-catalytic growth, solution treatment of copper foil, and thermal annealing of copper substrates (foils, grids, and wires) in air. [7][8][9][10][11][12][13][14] In the previous approaches, free-standing copper oxide nanowires or aligned nanowires on copper substrate have been successfully synthesized. However, no approach is found in the literature to grow vertically aligned copper oxide nanowires along silicon substrate. It is very interesting to realize copper oxide nanowires on silicon substrate that is a basic and widely used material in semiconductor industry. This will possibly provide the opportunities * Authors to whom correspondence should be addressed.

to integrate the nanowires into silicon based micro systems, thus resulting in nano based functional devices. Nano energetic material based high temperature initiators suggested in Refs. [15-16] are typical examples for these functional nano devices. Among the above-mentioned synthesis approaches of copper oxide nanowires, the simplest way is to heat copper substrates directly in air. [12][13][14] By employing the similar method, it maybe possible to grow copper oxide nanowires on copper thin film deposited onto silicon substrate. However, in accordance with the literature, [17][18] it seems very hard to realize metal oxide nanowires just from pure metal thin films by thermal annealing. Furthermore, the copper oxide nanowires synthesized in static air are not well vertically aligned on the top surface, 12 which is not desirable for some special applications. For instance, it is not favorable for the nano aluminum deposition to realize CuO/Al based nano energetic materials. 16 In this study, we successfully synthesized large-area and well vertically aligned pure CuO nanowires by thermal annealing copper thin film deposited onto silicon substrate under a N 2 /O 2 gas flow.

EXPERIMENTAL DETAILS

A 30 nm titanium film is first deposited onto a 4-inch silicon wafer followed by a 50 nm copper film deposition by thermal evaporation, where the titanium film acts as the doi:10.1166/jnn.2008.C1004 adhesion layer between copper and silicon substrate and the copper film serves as the electrical conducting layer for the subsequent electroplating. Copper film with a thickness of 1 m is deposited onto the substrate by electroplating. The substrate is cut into small chips that are then treated for 20 s in a solution containing 120 ml DI water and 10 ml 37% HCl. After being rinsed with DI water and blown-dried using N 2 , the chips are placed on a clean silicon wafer. The wafer is put onto a quartz boat that is positioned in the center of a quartz tube. The quartz tube is mounted inside a horizontal tube furnace. A high-purity N 2 flow at a rate of 2000 sccm is first introduced into the quartz tube for 20 min to remove air in the system. And then the N 2 flow is reduced into 400 sccm. Simultaneously, a high-purity O 2 flow at a rate of 100 sccm is introduced into the system. The tube furnace is heated to the set-point temperatures at about 8 C/min at atmospheric pressure. After being held for the designed time, the O 2 flow is terminated. And only N 2 is kept flowing. The furnace is cooled down naturally to room temperature. The chips are taken out of the furnace for further analysis. 2(A-B). As stated in Ref. [12], the copper oxide nanowires realized in static air from copper substrate are not well vertically aligned on the top surface. The vertically aligned CuO nanowires synthesized on silicon substrate in this study have some potential advantages over the CuO nanowires prepared on copper substrate under static air in Refs. [12-14]. A typical example is that the vertical nanowires are favorable for the subsequent nano aluminum deposition so as to achieve CuO/Al based nano

RESULTS AND DISCUSSION

SEM Characterization

ARTICLE

energetic materials. 16 Furthermore, since the nanowires are realized onto silicon substrate, it is easier to integrate them into silicon based micro systems. This will probably lead to CuO nanowire based functional devices. According to Ref. [12], CuO nanowires were formed in a temperature range from 400 to 700 C while copper substrates were annealed in static air. Nevertheless, in this study long and aligned CuO nanowires can only be synthesized in a narrow temperature range from 400 to 500 C when copper thin films are heated under a N 2 /O 2 gas flow. Below 350 C, no nanowires are formed and the thin film surface is mainly covered by nano particles. When the temperature is higher than 550 C, there is a very small amount of short nanowires formed on the thin film surface. Figures 3(A-B) shows the SEM images of the annealed copper film at 400 C for 5 h with different N 2 /O 2 gas flow rates. When the N 2 /O 2 gas flow rate is very low (100/25 sccm), the asprepared nanowires are not well vertically aligned along the substrate as shown in Figure 3(A), which is similar to those synthesized in static air. However, when the N 2 /O 2 gas flow rate is too high (800/200 sccm), only very short nanowires are obtained as can be seen in Figure 3(C). The N 2 /O 2 gas flow rate of 400/100 sccm found in the present study is a moderate value for realizing long and vertically aligned nanowires along the substrate.

XRD Characterization

Figure 4 shows the X-ray diffraction (XRD) pattern of the annealed copper film at 400 C for 5 h. According to the results in Refs. [12-13], both CuO and Cu 2 O are observed when copper substrates are thermally treated in static air. In this study only CuO is found after copper thin film is annealed under a N 2 /O 2 gas flow at 400 C for 5 h as can be seen in Figure 4. The deposited copper thin film is all converted into CuO in the form of both nanowires and thin film. As mentioned previously, a 30 nm thick Ti film is deposited onto silicon to serve as the adhesion layer between copper and silicon. After the thermal annealing, no Ti or TiO 2 is observed from the XRD pattern. This is mainly caused by the very small Ti film thickness. The obtained pure CuO is beneficial for realizing CuO/Al based nano energetic materials as suggested in Ref. [16] because the heat of reaction (4.144 kJ/g) between CuO and Al is much higher than that (2.434 kJ/g) between Cu 2 O and Al. [19][20] Vapor-liquid-solid and vapor-solid mechanisms are usually used to interpret the nanowire growth in the gas phase. 12 13 17 Vapor-solid mechanism is supposed to be accounted for the CuO nanowire growth because the particle-type termination of the nanowire, a most notable sign of the vapor-liquid-solid growth mechanism does not present in accordance with our SEM observations. As mentioned previously, the CuO nanowires synthesized in this study under a N 2 /O 2 gas flow appear to be more vertical, denser, and thicker compared to the CuO nanowires realized in static air in Refs. [12-14]. When static air is used, the O 2 concentration decreases with the reaction near the thin film surface because the oxidation rate is larger than the diffusion rate. The locally low O 2 concentration is not beneficial for the nanowire growth. Employing gas flow increases the diffusion rate and the oxidation is no more limited by diffusion.

TEM Characterization

Figure 5 displays the high resolution transmission electron microscopy (HRTEM) images of an individual nanowire grown from the annealed copper thin film at 400 C for 5 h. It can be seen that each side of the nanowire is a single crystal with a distinct fringe space pattern. The interplanar spacing for each side is 1.98 Å and 2.35 Å, respectively, which is similar to the spacing for {111} and { 111} planes in monoclinic CuO. This confirms the bicrystallinity of the as-prepared nanowires.

CONCLUSIONS

In this study, pure and aligned CuO nanowires have been successfully realized onto silicon substrate. The nanowires are synthesized by annealing copper thin film deposited onto silicon under a N 2 /O 2 gas flow. Compared to the CuO nanowires grown from copper substrates in static air performed by previous approaches, the nanowires achieved here are more vertical, purer, and more uniform. Therefore, they are favorable for subsequent nano Al deposition so as to realize CuO/Al based nano energetic materials. Long and aligned CuO nanowires can only be formed in a narrow temperature range from 400 to 500 C. A moderate high N 2 /O 2 gas flow rate of 400/100 sccm can help prepare well vertical and thicker nanowires along the copper thin film because the gas flow is able to keep a relatively high O 2 concentration near the thin film to support the nanowire growth. XRD and HRTEM results indicate that the entire copper thin film is converted into pure CuO in terms of both bicrystal nanowires and thin film. Because the nanowires are realized onto silicon substrate, it is supposed to be easier to integrate the nanowires into silicon based micro systems, thus probably resulting in nanowire based functional devices.

Fig. 1 .

 1 Fig. 1. 30 tilted view SEM images of the annealed copper film at 400 C for 5 h.

Figure 1 (

 1 Figure1(A) shows a 30 tilted view scanning electron microscopy (SEM) image of the annealed copper film at 400 C for 5 h. The diameters of the as-realized CuO nanowires are in the range of 60-100 nm. The nanowires in Figure1(A) are shown in Figure1(B) with a lower magnification. It can be seen that large-area and uniform nanowires are grown from the copper thin film surface after the thermal treatment under a N 2 /O 2 flow. The resulting nanowires are mostly vertically aligned along the film surface. This is confirmed by the top view SEM images in Figures2(A-B). As stated in Ref.[12], the copper oxide nanowires realized in static air from copper substrate are not well vertically aligned on the top surface. The vertically aligned CuO nanowires synthesized on silicon substrate in this study have some potential advantages over the CuO nanowires prepared on copper substrate under static air in Refs.[12-14]. A typical example is that the vertical nanowires are favorable for the subsequent nano aluminum deposition so as to achieve CuO/Al based nano

Fig. 2 .

 2 Fig. 2. Top view SEM images of the annealed copper film at 400 C for 5 h.

Fig. 3 .

 3 Fig. 3. 30 tilted view SEM images of the annealed copper film at 400 C for 5 h in (A) N 2 /O 2 (100/25 sccm), (B) N 2 /O 2 (400/100 sccm), and (C) N 2 /O 2 (800/200 sccm).

Fig. 4 .

 4 Fig. 4. XRD pattern of the annealed copper film at 400 C for 5 h.

Fig. 5 .

 5 Fig. 5. HRTEM images of an individual nanowire from the annealed copper film at 400 C for 5 h.