Transferable Highly (001) Oriented Sb2Se3 Nanorod Films on Flexible Substrates for Innovative Optoelectronic Devices

Thin films of antimony chalcogenides, Sb2X3 (X = S, Se, or SxSe1−x), comprised of (001) oriented nanorods are highly desirable for optoelectronic applications owing to their light trapping capacity and highly efficient charge transport along the optimally aligned quasi‐1D (Sb4X6)n ribbons. Here, highly (001) oriented Sb2Se3 nanorod (Sb2Se3NR) layers are obtained via a novel template growth method, and transfer these layers to functional substrates via a polymer‐assisted technique. Transferable layers overcome the current challenges associated with achieving oriented growth directly on the desired substrate. Using a combination of a low‐temperature processable SnO2 nanoparticle transport layer and encapsulation/etching techniques photodetector devices are fabricated with excellent electrical contact. The optimized transferred devices show broad range photoresponse with signal‐to‐noise ratios up to 105 and responsivities reaching 0.96 mA W−1. Importantly, access to low‐temperature processing enables the fabrication of flexible devices. Based on this platform, a proof‐of‐concept self‐powered flexible heart rate monitor is fabricated. The achievement of transferable Sb2Se3NR layers introduced in this work is expected to be readily applicable to generate new (001) Sb2Se3 device architectures that may otherwise not be achievable via direct deposition, with application in photoelectrochemical water splitting, battery electrodes, and solar cells.


Introduction
Antimony chalcogenide (Sb 2 Se 3 , Sb 2 S 3 or Sb 2 (S,Se) 3 ) thin films have achieved extraordinary performances over a wide range DOI: 10.1002/admi.202300604 of electronic device applications including photoelectrochemical cells, batteries, solar cells, and PDs. [1]Sb 2 Se 3 has garnered particular interest for use as solar cell absorbers, with conversion efficiencies recently exceeding the 10% benchmark. [2,3]Solar cells, and photoelectrochemical cells, made with Sb 2 Se 3 often have a remarkably high short-circuit current (J SC ), close to 30 mA cm −2 , [4] and Sb 2 Se 3 battery electrodes show high performance (> 450 mA h g −1 at > 20 A g −1 ). [5]Some key optoelectronic properties of Sb 2 Se 3 for device applications include a direct band gap of 1.2 eV, [6] high optical absorption coefficient (>10 −5 cm −1 ), [7] as well as good electron and hole mobility (≈15 and ≈40 cm 2 V −1 s −1 respectively), [8] making it highly appealing for incorporation into a range of optoelectronic devices. [9]ntimony selenide has a lowdimensional crystal structure composed of (Sb 4 Se 6 ) n quasi-1D ribbons that extend along the [001] crystallographic direction of the orthorhombic (Pbnm) crystal structure. [10]Due to Sb 2 Se 3 having low crystal symmetry, crystallographic orientation control is crucial for optimal device efficiency.A perfectly (001) oriented Sb 2 Se 3 active layer is the most desirable for electronic device applications as charge transport is vastly more efficient along the covalently bound ribbons. [11]However, (001) oriented Sb 2 Se 3 films are not readily attainable.Sb 2 Se 3 film growth is especially sensitive to substrate type, i.e., films deposited under the same conditions but on different substrates will present completely distinct crystallographic alignment and morphology. [12]Orientation control is further complicated by the tendency of ribbons to grow in less desirable (hk0) orientation, exposing the lower energy Van der Waals planes on the surface of the substrate. [13]Although methods to grow optimally oriented Sb 2 Se 3 films have been extensively researched, [2,14] obtaining films with a high degree of (001) orientation still requires extensive optimization of deposition parameters for each different substrate -and may not always be possible.Thus, a method for obtaining a (001) preferred orientation in Sb 2 Se 3 films on a range of functional substrates is highly attractive.
Film transfer can circumvent the synthetic challenges related to achieving oriented growth directly onto the desired substrate. [15]Transferring is especially useful to fabricate flexible electronic devices with the possibility of nanoscale functional features or large patterned areas. [16]The active layer is first grown on a donor substrate (template) that provides the conditions for the growth of high-quality films.The film may then be transferred to the receiving substrate.Polymer-assisted transfer techniques are commonly employed to fabricate devices with quasi-2D functional layers. [17]The layer is immobilized in a polymer matrix and then detached from the template mechanically, hydrophobically, or by dissolving an interlayer. [18]The basic requirement is that the immobilized layer is stable enough to withstand the mechanical stress of "lift-off" from the donor substrate. [19,20]The detached layer can then be transferred directly to a receiving substrate to build the device.Importantly, transfer techniques decouple growth requirements for donor and receiving substrate and allow for the stacking of (oriented) layers into architectures that would be impossible to achieve via direct deposition.Although methods to transfer 2D layers have been well researched, [18] the ability to effectively transfer large-area bulk layers (especially nanostructured layers) is still underdeveloped.Access to practical fabrication methods, tailored to preserve the integrity and alignment of crystal grains on the receiving substrates, is required to manifest the full potential of (001) Sb 2 Se 3 nanorod thin films.
Here we show for the first time a robust method to transfer (001) preferred orientated Sb 2 Se 3 nanorod (Sb 2 Se 3 NR) thin films.Through the use of an optimized back etching process, we provide electronic access to the transferred film and fabricate functional photodetectors (PDs) and a proof-of-concept medical device (flexible self-powered heart rate monitor).The template film is grown on a donor substrate optimized for (001) Sb 2 Se 3 NR growth, immobilized in PMMA, detached in aqueous solution, and transferred to rigid or flexible substrates.X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) show that the crystallographic orientation, chemical composition, and grain morphology are all preserved after transfer.PD performance is investigated as a function of electron transport interlayer for a variety of receiving substrates.Finally, a wearable heart rate detector is demonstrated by transferring the Sb 2 Se 3 NR layer onto a flexible PET/ITO substrate.

Template Growth of the (001) Sb 2 Se 3 Layer
Recently, we demonstrated that a substrate's physical nature (morphology and nanostructure) can be tailored to direct the crystal orientation and morphology of evaporated Sb 2 Se 3 films. [21]An appropriately nanostructured substrate (ZnO nanowires (ZnO-NW)) can drive the growth of highly (001) oriented Sb 2 Se 3 NR layers via an "orientation filtering" mechanism, wherein (hk0) crystals are trapped by the ZnO-NW substrate in the early stages of growth.This yields an Sb 2 Se 3 layer with a very high density of vertically aligned (001) oriented crystals.Figure 1a shows the crosssection scanning electron microscope (SEM) image of a typical Sb 2 Se 3 NR layer obtained via this method highlighting the array of vertically aligned Sb 2 Se 3 nanorods.The detailed procedure is described in the experimental section.In short, a chemical bath deposition (CBD) method [22] was used to obtain the ZnO-NW on glass/FTO, and the Sb 2 Se 3 was deposited via vapor transport deposition (VTD). [23]s summarized in Figure 1b, film transfer was achieved using a polymer-assisted technique.First, the Sb 2 Se 3 NR film is immobilized in a layer of polymethylmethacrylate (PMMA) (deposited via spin coating).The PMMA layer was optimized to ensure the polymer infiltrated the gaps between the Sb 2 Se 3 NRs and just covered their tips.The sample is then immersed in a 1 m H 2 SO 4 solution to dissolve the ZnO-NW underlayer and detach the active Sb 2 Se 3 layer.Although the Sb 2 Se 3 NR layer by itself (on the template) is fragile, the Sb 2 Se 3 NR/PMMA composite forms a fairly robust membrane.The detached "floating" Sb 2 Se 3 NR layer encapsulated in the PMMA film is then moved to a clean water bath to wash out the excess acid, followed by being transferred to a new substrate (via underside lift-off directly from the water bath), and finally dried on a hot plate.The transfer method described here is highly effective, but care must be taken during transferring as the detached film may curl.(Figure S1, Supporting Information).Although applied to ZnO-NWs, this transfer method is directly applicable to both compact and nanorod films on any "dissolvable" underlayer.
Figure 2a shows cross-section SEM images of the Sb 2 Se 3 /PMMA layer on the ZnO-NW template before and after transferring to a glass/FTO substrate.The SEM images reveal that the Sb 2 Se 3 -NR remains completely intact after transferring to the receiving substrate.Importantly, the vertical alignment of the nanorods is well preserved.To further assess the effectiveness of our transfer method, the crystallographic orientation of the Sb 2 Se 3 -NR was investigated before and after transfer.Figure 2b shows the XRD patterns for the template and transferred Sb 2 Se 3 NR layers.A high intensity (002) reflection is observed for the template thin film, which is indicative of the exceptional vertical alignment of Sb 2 Se 3 grains in the <001> crystallographic direction.It can be seen that, after transfer, the intensity of the (002) reflection is preserved.Texture coefficient analysis is the most accurate way to elucidate any fine changes in crystallographic orientation and thus the effectiveness of the substrate transfer.Figure 2c shows the texture coefficient (TC) for key (hkl) reflections before and after transfer.Using our optimized transfer method, a remarkable 80% of (002) texture is retained on the receiving substrate (TC (002) = 12.7 before and 10.1 after), while the (101) texture increases by ≈10% (TC (101) = 4.5 before and 4.8 after).These data highlight the efficacy of the transfer method to preserve crystallographic alignment.The full set of analyzed TCs is shown in Figure S2 (Supporting Information).For reference Figure 2d shows a picture of the Sb 2 Se 3 NR film on a receiving glass/FTO substrate.The Sb 2 Se 3 NR layer is seen to be matte black due to its light-trapping capacity, which helps to harvest incident light more effectively.The transferred film has a remarkably low reflectance of only 2.4% across the entire visible range (Figure S3, Supporting Information).Close inspection confirms that the transferred layer is uniform, flat, and well adhered to the substrate providing an excellent platform for the development of functional electronic devices.

Sb 2 Se 3 NR Photodetectors from Transferred Layers
Our transfer method is highly effective at relocating sensitive thin films with preservation of initial morphology, however, PMMA is not conductive.As such, to fabricate the optoelectronic devices from transferred layers it is necessary to make the films electronically accessible before depositing a back electrode material.To achieve this, clean Sb 2 Se 3 surfaces must be obtained by partially etching the PMMA from atop the encapsulated Sb 2 Se 3 NR thin film exposing only the tips of the NRs, as shown in Figure S4 (Supporting Information).Figure 3a shows that this can be achieved by treating the film under O 2 plasma such that electrode deposition occurs onto the freshly exposed Sb 2 Se 3 NR tips. Figure 3b shows the backscattered electron SEM (BS-SEM) images of the encapsulated Sb 2 Se 3 NR/PMMA film (before plasma) and the partially etched film (after plasma).Before O 2 plasma treatment, the relatively low atomic weight PMMA film produces a low BS-SEM signal, yielding darker regions in the image.After plasma etching, the exposed Sb 2 Se 3 -NR scatter more electrons due to their higher atomic weight and thus produce a stronger BS-SEM signal, resulting in brighter regions.The topview secondary electron SEM before and after transfer are shown in Figure S5 (Supporting Information) and EDS analysis of the initial (as-deposited) film and the final (etched and transferred) film are shown in Figure S6 (Supporting Information).EDS analysis confirmed that there is little change to the bulk Sb:Se atomic ratio after the etching process.
The effectiveness of the etching process to expose Sb 2 Se 3 surfaces was also investigated using XPS (Figure 3c).The survey XPS spectrum before plasma etching (dark grey spectrum) shows O1s and C1s peaks characteristic of PMMA at 532 eV and 585 eV, respectively. [24]Characteristic peaks for Sb3s, Sb3p, Sb3d, and Se3d are only clearly discernable once the Sb 2 Se 3 NR are exposed after plasma etching the PMMA (red spectrum).These results confirm two key points: i) that Sb 2 Se 3 -NR are well encapsulated by a PMMA film before plasma treatment, which is valuable as the PMMA layer helps isolate the Sb 2 Se 3 -NR from the acid bath used for the transfer process; and ii) that the O 2 plasma treatment is effective to etch the PMMA and expose the Sb 2 Se 3 NR tips.High-resolution XPS analysis of the Sb and Se regions on the template and transferred layers (Figure S7, Supporting Information) confirm that Sb 2 Se 3 remains chemically stable during the transferring process, although a layer of Sb and Se oxides is formed upon O 2 plasma treatment.The presence of Sb-O has been shown to reduce device performance, [25] although a very thin layer (no more than ≈2 nm) of Sb 2 O 3 may help to suppress charge recombination at the back contact. [26]Should this be of concern, it is possible that further improvements may be realized upon post treatments to remove surface oxides, or using milder plasma etching conditions (i.e., with Ar gas).
Having established electronic access at the top Sb 2 Se 3 contact we turn our attention to the back substrate contact.In Sb 2 Se 3 optoelectronic devices, the quality and chemical composition of this contact is known to strongly affect device performance. [23,27]ikewise, the type of contact is expected to affect the adherence of the transferred film.To evaluate different substrate contacts, prototype PD devices were developed (Figure 4a) with a solution-processed carbon paint back electrode.Substrates were chosen based on a direct transfer to a conductive FTO substrate (control devices), as well as common n-type contacts (electron transport layers, ETLs) known to be applicable to Sb 2 Se 3 devices and to have adequate band alignment (Figure S8, Supporting Information).Specifically, PD devices were fabricated by transferring the Sb 2 Se 3 NR onto glass/FTO, glass/FTO/CdS, and glass/FTO/SnO 2 .Figure 4b-d shows SEM images of the films on each of these layers, highlighting the intimate contact of the Sb 2 Se 3 NR thin film to each substrate after transfer.The performance of these devices was first evaluated as a function of annealing temperature on each substrate to find the optimal temperature for substrate adherence and electrical contact.It was found that relatively independent of substrate, devices annealed at 175 °C provided the best performance (see Figures S9 and S10, Supporting Information).
An important and desirable feature of PD devices is their ability to work independently of an external power source.Figure 4e shows the photocurrent density (J ph ) of the self-powered (0 V bias) PDs on each substrate, and the J-V curves are plotted in Figure S11 (Supporting Information).The device with a SnO 2 ETL (108 μA cm −2 ) greatly outperforms devices with a CdS ETL (10.9 μA cm −2 ), as well as devices constructed directly on FTO (18.8 μA cm −2 ).This indicates that SnO 2 is the preferred ETL for application.Notably, the transferred PDs with a SnO 2 ETL show a J ph only 7% lower than the initial control device from direct evaporation (108 and 116 μA cm −2 respectively).These data highlight the excellent performance of the front contact given that interface contact (and hence photocurrent) is expected to be best when the Sb 2 Se 3 is directly evaporated onto a substrate.
The performance of PDs was further characterized by calculating the signal-to-noise ratio (SNR = I photo /I dark ) and the responsivity (R) of the devices. [28]R (A W −1 ) is a measure of the PD efficiency and is calculated by the ratio between input power at a given light wavelength (P  ) and output current density and is given by: where S is the active area of the PD, and I ph is the photocurrent (light current minus dark current).Figure 4f shows R for the PDs built on transferred layers and the (as-deposited) template control.These devices show excellent SNR switching ratios approaching ≈10 5 , providing a large (signal) scope for targeted device applications, and is especially remarkable considering that Sb 2 Se 3 layer is transferable and can be built on flexible substrates (vide infra).Notably, the photodetector (Sb 2 Se 3 NR layer) retained high-level performance after transfer to optimal FTO/SnO 2 substrate, yielding an R within ≈10% (0.96 mA W −1 vs 1.04 mA W −1 ).Collectively, these results show that intimate electrical contact can be formed using a SnO 2 interface and that the electronic properties of the Sb 2 Se 3 NR film are well preserved after transfer.
To evaluate the extraction of photogenerated carriers in transferred devices with and without an ETL, the response to frequency-modulated light was assessed.Figure 4g   considerably faster (−3 dB of 13 kHz), which indicates that the junctions formed in the devices from transferred layers can still be improved.Annealing at higher temperatures typically provides more intimate contact, however, they may cause the Sb 2 Se 3 to degrade (Figure S14, Supporting Information).Moreover, high annealing temperatures preclude the fabrication of flexible devices.

Photodetectors and Heart Rate Monitor
Engineering flexible thin-film devices present unique challenges.First, the device must be able to be fabricated at mild temperatures given that most flexible substrates have a far lower heat resistance compared to glass.As we have shown our devices can successfully be processed at temperatures (150-175 °C) applicable to flexible device fabrication with common commercially available polyethylene terephthalate (PET) substrates. [29]The second main requirement is that all layers must withstand the shear stress caused by flexing, without fracturing or detaching from the substrate. [30]][33] Sb 2 Se 3 has lower Young's modulus along the crystallographic a-and b-axis because ribbons are held together only by weak van der Walls forces along these directions. [33]As such, vertically oriented Sb 2 Se 3 films that can more easily deform on the ab-plane present a great advantage for flexible device applications.However, if ribbons were laid horizontally on the substrate, [32] tension is applied directly along the strong covalent bonds within the (Sb 4 Se 6 ) n chain when the sub-strate flexes.Furthermore, a typical compact film (with densely interconnected grains) may also struggle to accommodate tension, as forces act to pull grains apart upon flexing.A (001) Sb 2 Se 3 layer composed of vertically oriented nanorods is far more suitable to endure large deformation stress as schematically represented in Figure S16a (Supporting Information).As such, transferrable (001) oriented Sb 2 Se 3 NR layers provide an avenue to fabricate robust flexible devices.
Here, flexible devices were fabricated by transferring the Sb 2 Se 3 NR layers onto PET/ITO/SnO 2 substrates.Figure 5a shows the device upon flexing, highlighting that the transferred Sb 2 Se 3 NR remains well attached to the substrate.We note that optimal performance was obtained at ≈175 °C.Lower temperatures (annealed at 80 °C) presented both poor electronic and mechanical performance (Figure S15, Supporting Information).To assess the electronic performance of the optimized device upon mechanical stress, the PD was exposed to numerous flexing cycles and flexed at increasing angles, , (Figure S16b, Supporting Information).Stable performance (within 3% deviation) is demonstrated over 250× flexing cycles.The device also shows great mechanical stability and no loss in photocurrent even at very high flexing angles (Figure 5b).In fact, a slight increase in J ph (≈20%) upon flexing was observed.Furthermore, Figure 5c reveals that our self-powered flexible device is stable over light cycling.
The photoresponse of the flexible detector was further characterized as a function of light irradiance.Fitting these data to a power law function allows important information to be extracted on device dynamics.Devices with power law exponents ≪ 1 indicate the presence of traps at detrimental locations in the device, either at interfaces or in the bulk transport layer. [34]Devices fabricated directly on ITO have a low current/power ratio, exhibiting a J ph ≈ P 0.54 , and thus possess a significant density of trap states.In contrast, devices with a SnO 2 interlayer have near unity power law exponent (J ph ≈ P 0.96 ) indicating the passivation (removal) of traps residing predominantly at the ITO/Sb 2 Se 3 NR interface.Consequently, the device exhibits a linear dynamic range (LDR) measured here to over 3 orders of magnitude in irradiance.These data highlight that the Sb 2 Se 3 NRs themselves are highly crystalline with few defects -otherwise current would scale sub-linearly regardless of interface passivation.The flexible device shows reasonable R and SNR values, up to 0.54 mA W −1 and ≈10 4, respectively, and rapid response times with a 3 dB cutoff at 118 Hz.Notably, the flexible PDs showed both a fast and a slow signal decay component.Although OFF/ON switching can exceed 10 5 the slow decay in photocurrent due to trapped carriers (capacitance) limits the time response (and 3 dB) of the devices (Figure S17, Supporting Information).Collectively, the physical robustness of these devices and their flexibility, coupled with their excellent preliminary performances provide a sound platform for key commercial applications.
Flexible PDs may be used to fabricate wearable heart rate monitors. [33,35]Figure 6a illustrates the working principle of a heart rate photosensor. [36]When biological tissue is illuminated by a light source, most of the light is scattered or absorbed in the blood and other tissues.The amount of light absorbed in arterial blood varies during the cardiac cycle due to changes in the blood volume in the arteries.Light can be easily transmitted through biological tissue at body extremities (e.g., fingertips or earlobes) and sensed by an appropriate PD.Transmission is at a minimum when the heart contracts pumping blood to the arteries (systole), and at a maximum when the heart expands pulling blood from arteries (diastole).As shown in Figure 6b, hemoglobin in the blood absorbs light in the visible and NIR range, which is well within the Sb 2 Se 3 absorption range. [37]As our flexible detector performs well within the 455 -730 nm spectral range (Figure S18, Supporting Information), it is suitable to probe the variation of blood volume during the cardiac cycle (pulse).
To probe the heart pulse rate, a red LED was used as the illumination source.Figure 6c shows our flexible PD wrapped around the finger, the percentage of red light that is transmitted through the finger is clearly visible in the high-contrast image.Figure 6d shows the detector response over time when placed over an individual's finger.The detector senses the transmitted light, and the electrical current output is measured as a signal that is plotted over time.The signal is maximum during diastole when transmitted light is maximum (low blood volume in the arteries) and minimum during systole when transmitted light is minimum (high blood volume in the arteries).The pulse rate can be gathered immediately, which will update after each subsequent heartbeat (peak in photocurrent).In this case, the pulse rate was determined to be ≈48 beats per minute (bpm).Ongoing monitoring (after ≈10 s) provides enough data to determine the pulse rate more accurately.Figure 6e shows the fast Fourier transform (FFT) of the signal with a clear peak of an easily distinguishable signal-to-noise ratio at a frequency of 0.8125 Hz or 48.75 bpm.The detector also showed excellent performance under lower energy infrared light (730 nm) illumination (Figure S19, Supporting Information).These results clearly show that our robust and flexible PD, integrated with a low-energy red or infrared light source, may be used to accurately monitor heart (pulse) rate.

Conclusion
A template growth method combined with an optimized polymer-assisted transfer technique has been demonstrated as an effective way to obtain highly (001) oriented Sb 2 Se 3 NR layers on any desired substrate.Using our method, the exceptionally high (001) orientation is preserved after transferring to the receiving substrate.The methods developed here are robust and remove the substrate-dependent constraints previously encountered to obtain oriented Sb 2 Se 3 films by direct deposition.To demonstrate the applicability of this process to optoelectronic devices, PDs were fabricated, which showed excellent performance.Optimized transfer and PMMA back-etching processes yielded transferred FTO/SnO 2 /Sb 2 Se 3 NR devices having <10% current loss compared to the control (not transferred) devices.To demonstrate the versatility of this transfer process and the high quality of the Sb 2 Se 3 NR films both flexible PDs and a proof-of-concept flex-ible heart rate monitor were fabricated by transferring Sb 2 Se 3 NR onto a PET/ITO substrate.The ability to transfer delicate (001)-Sb 2 Se 3 nanorod thin films onto any substrate, preserving morphology and orientation, represents an important development toward other innovative device architectures.It is expected that the methodology outlined here may be used to fabricate highly efficient solar cells (especially those in substrate device configurations) as well as photoelectrochemical cells, and micro batteries.
ZnO-NW: The nanostructured ZnO layer was deposited via chemical bath deposition (CBD) on glass/FTO substrates seeded with a thin ZnO layer. [22]First, substrates were cleaned in soapy water, acetone, and IPA ultrasonic bath for 10 min each.ZnO seeds were deposited via a sol-gel method by adding ethanolamine (105 μL) to 0.35 m zinc acetate dihydrate in 2-methoxyethanol (5 mL).The mixture was left to stir for ≈2 h until a completely clear solution was obtained and filtered through a 0.22 μm PTFE syringe filter.This sol-gel solution was spun on clean glass/FTO substrates at 4000 RPM for 30 s. Two layers (≈30 nm) were deposited to ensure full coverage.Substrates were heated at 200 °C for ≈2 min between layers before final annealing at 250 °C for 60 min in air.To deposit ZnO-NW the seeded substrates were inclined inside a small (≈25 mL) beaker with the reactive (seeded) surface facing slightly downwards.Aqueous Zn(NO 3 ) 2 (20 mM in ≈0.8% NH 3 ) was then added to cover the substrate.The beaker was then placed in a water bath at 80 °C for exactly 60 min until deposition was stopped by transferring substrates to a milli-q water bath.Films were then dried under an N 2 gas stream.All ZnO-NW substrates were annealed at 500 °C for ≈1 h before being used for Sb 2 Se 3 deposition.
Sb 2 Se 3 Deposition: An optimized vapor transport deposition (VTD) method was used to deposit the Sb 2 Se 3 films, as previously described. [23]n short, the Sb 2 Se 3 source (quartz crucible with Sb 2 Se 3 powder) was put at the center of the left heating zone of a dual-zone furnace (equipped with a quartz tube).The mass of Sb 2 Se 3 powder in the crucible was 2000 mg.Substrates were mounted within the right heating zone on a near vertical graphite holder positioned 10 cm from the furnace center.Pressure was equilibrated at 0.5 -0.7 Pa before starting the heating program.Both zones were then heated at 250 °C for 30 min.The left zone, containing the Sb 2 Se 3 source, was then heated to 540 °C over the course of 15 min (≈20 °C min −1 ) and the temperature was held at 540 °C for 10 min.The right zone, containing the substrates, was kept at 250 °C throughout the whole process.The furnace was left to cool naturally for ≈2 h before ambient pressure was restored by bleeding N 2 gas into the tube.For heart rate monitoring, the device was placed on the outside of the finger, which was then illuminated (through the finger) with a 625 nm LED (100 W cm −2 ).The signal was monitored over time via a Keithley 2636B SourceMeter.The volunteer gave written informed consent for the experiment.
Instrumentation: The scanning electron microscope (SEM) images and backscattered SEM (BS-SEM) images were taken on a FEI Verios 460L XHR-SEM equipped with in-lens secondary electron detector (TDL) and in-column backscattered detector (ICD).X-ray diffraction (XRD) patterns were obtained using a Bruker AXS D4 Endeavour diffractometer with a Cu-K radiation source.Reflectance spectra were collected on a Cary 7000 UV-Vis-NIR spectrophotometer equipped with an integrating sphere.Xray photoelectron spectra (XPS) were collected on a Thermo Scientific Kalpha XPS instrument equipped with a monochromated Aluminium K source (1486.7 eV).

Figure 1 .
Figure 1.a) Scanning electron microscope (SEM) cross-section image of Sb 2 Se 3 NR on ZnO-NW and b) scheme of transferring process from ZnO-NW to a new substrate.The scale bar in the SEM corresponds to 1 μm.

Figure 2 .
Figure 2. a) Scanning electron microscope (SEM) images of ZnO-NW/Sb 2 Se 3 NR/PMMA (template) and Sb 2 Se 3 NR/PMMA transferred onto FTO.Scale bars in the SEM images correspond to 1 μm; b) X-ray diffraction (XRD) pattern highlighting the relative intensity of (002) reflection and c) texture coefficient of Sb 2 Se 3 for selected reflections before and after transfer; d) photograph of a transferred Sb 2 Se 3 NR layer on a glass/FTO substrate.

Figure 3 .
Figure 3. a) Scheme showing device preparation: O 2 plasma treatment used to partially back-etch the PMMA to expose the tips of Sb 2 Se 3 NR, followed by drop-casting the back contact electrode directly on the exposed Sb 2 Se 3 NR tips.b) Top-view backscattered scanning electron microscope (BS-SEM) images and c) corresponding survey x-ray photoelectron spectra (XPS) of Sb 2 Sb 3 NR before and after O 2 plasma exposure.Scale bars correspond to 1 μm.
reveals that the −3 dB bandwidth of devices with a SnO 2 ETL was ≈3× higher (73.7 Hz) compared to the device with FTO only (22.5 Hz) or FTO/CdS (25.8 Hz).The full curves and rise/fall times are plotted in Figures S12 and S13 (Supporting Information).This indicates that the photogenerated carriers are extracted more effectively by the SnO 2 ETL.It is worth noting that the control device was

Figure 4 .
Figure 4. a) Schematic representation of the stacked layers in the devices.Cross-section SEM images of Sb 2 Se 3 NR/PMMA layer transferred onto b) glass/FTO, c) glass/FTO/CdS, and d) glass/FTO/SnO 2 .Comparison of e) photocurrent density (J ph ) and f) Typical Responsivity (R) and signalto-noise ratio (SNR) for as-deposited (template) devices and transferred devices on different substrate types.g) 3 dB cut-off frequencies for transferred devices.A 625 nm LED at 115 mW cm −2 was used as the illumination source for photocurrent measurements.Scale bars correspond to 500 nm.

Figure 6 .
Figure 6.a) Scheme highlighting the light transmitted through the biological tissue as the blood volume in the arteries change; b) absorption spectrum of Sb 2 Se 3 NR and hemoglobin in its oxygenated (HbO 2 ) form; [37] c) flexible Sb 2 Se 3 NR heart rate detector wrapped around the finger; d) signal from pulse measurements and c) fast Fourier transform (FFT) of the pulse signal.A 625 nm LED at 100 W cm −2 was used as illumination source.
Sb 2 Se 3 Film Transfer: PMMA solution (10% in chlorobenzene) was spun on glass/FTO/ZnO-NW/Sb 2 Se 3 NR substrates at 1000 RPM for 60 s and dried at 80 °C for 10 min.The glass/FTO/ZnO-NW/Sb 2 Se 3 NR/PMMA substrate was put in a 1 m H 2 SO 4 bath overnight until the ZnO-NW layer was completely dissolved and the Sb 2 Se 3 NR/PMMA film detached from the substrate.The detached film was moved through two clean milli-q water baths to wash out the acid.The floating Sb 2 Se 3 NR/PMMA film was transferred to the receiving substrate directly from the water bath and dried on a hot plate at 80 °C for ≈30 min.PD Fabrication and Measurements: The encapsulated Sb 2 Se 3 NR/PMMA layers were etched in O 2 plasma for 90 s using a Plasma Etch benchtop system (PE-50), 15 cc min −1 O 2 flow.The C-paint or PEDOT back contacts were drop cast directly on the etched layers and dried for ≈10 min at 70 °C.Before deposition, the PEDOT solution was doped with 5% DMSO to increase conductivity.Photoresponse was measured on a customized setup with Keithley 2636B SourceMeter.Thorlabs LEDs 455, 530, 625, 730, and 850 nm were used as illumination sources.Rise/Fall times and −3 dB curves were obtained using function generator Stanford Research Systems (model DS345), low-noise current preamplifier Stanford Research Systems (model SR570), ThorLabs laser diode control (model LDC205C) with a 530 nm laser diode and oscilloscope Tektronix (TDS1012B).