Liquid Phase Exfoliated Indium Selenide Based Highly Sensitive Photodetectors

FULL PAPER www.afm-journal.de Liquid Phase Exfoliated Indium Selenide Based Highly Sensitive Photodetectors Nicola Curreli, Michele Serri, Davide Spirito, Emanuele Lago, Elisa Petroni, Beatriz Martín-García, Antonio Politano, Bekir Gürbulak, Songül Duman, Roman Krahne, Vittorio Pellegrini, and Francesco Bonaccorso* Layered semiconductors of the IIIA–VIA group have attracted considerable attention in (opto)electronic applications thanks to their atomically thin structures and their thickness-dependent optical and electronic properties, which promise ultrafast response and high sensitivity. In particular, 2D indium selenide (InSe) has emerged as a promising candidate for the realization of thin-film field effect transistors and phototransistors due to its high intrinsic mobility (>102 cm2 V−1 s−1) and the direct optical transitions in an energy range suitable for visible and near-infrared light detection. A key requirement for the exploitation of large-scale (opto)electronic applications relies on the development of low-cost and industrially relevant 2D material production processes, such as liquid phase exfoliation, combined with the availability of high-throughput device fabrication methods. Here, a β polymorph of indium selenide (β-InSe) is exfoliated in isopropanol and spray-coated InSe-based photodetectors are demonstrated, exhibiting high responsivity to visible light (maximum value of 274 A W−1 under blue excitation 455 nm) and fast response time (15 ms). The devices show a gate-dependent conduction with an n-channel transistor behavior. Overall, this study establishes that liquid phase exfoliated β-InSe is a valid candidate for printed high-performance photodetectors, which is critical for the development of industrial-scale 2D material-based optoelectronic devices. N. Curreli, Dr. M. Serri, Dr. E. Lago,[++] E. Petroni,[++] Dr. B. Martín-García, Dr. A. Politano,[+++] Dr. V. Pellegrini, Dr. F. Bonaccorso Graphene Labs Istituto Italiano di Tecnologia via Morego 30, 16163 Genova, Italy E-mail: francesco.bonaccorso@iit.it N. Curreli Dipartimento di Ingegneria Elettrica ed Elettronica Università di Cagliari P.zza d’Armi, 09123 Cagliari, Italy The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201908427. [+]Present address: IHP—Leibniz-Institut für innovative Mikroelektronik, Im Technologiepark 25, 15236 Frankfurt (Oder), Germany [++] Present address: STMicroelectronics, SMART POWER Technology R&D, Via C. Olivetti 2, 20864 Agrate Brianza, Italy [+++]Present address: Dipartimento di Scienze Fisiche e Chimiche, Università degli Studi dell'Aquila, Via Vetoio, 42, 67100 Coppito, Italy DOI: 10.1002/adfm.201908427 Adv. Funct. Mater. 2020, 30, 1908427 1. Introduction 2D semiconductor materials, including layered transition metal dichalcogenides (TMDs) with formula MX2 (M = Mo, W; X = S, Se, Te), have been intensely studied as substitutes for conventional semiconductors in opto-electronic applications.[1,2] The β polymorph of indium selenide (β-InSe) is a promising van der Waals layered material due to its remarkable fundamental properties,[3] such as a low electron effective mass (m* = 0.143 m0)[4] and a bandgap ranging from ≈1.26 eV for the bulk to ≈2.11 eV for the monolayer at room temperature,[5,6] crossing over from an indirect to a direct semiconductor when the crystal thickness is larger than six layers (≈5 nm, each layer is ≈0.84 nm thick).[7–9] The fundamental properties of β-InSe indicate the potential use of few-layer flakes in field effect transistors (FETs) and sensitive electric light detectors thanks to a direct bandgap, which is tunable by the crystal thickness in a wide spectral range from visible to Dr. D. Spirito,[+] Dr. R. Krahne Nanochemistry Department Istituto Italiano di Tecnologia Via Morego 30 ,16163 Genova, Italy Dr. E. Lago, E. Petroni Dipartimento di Chimica e Chimica Industriale Università degli Studi di Genova via Dodecaneso 31, 16146 Genoa, Italy Dr. B. Gürbulak Department of Physics Faculty of Sciences Atatürk University 25240 Erzurum, Turkey Prof. S. Duman Department of Basic Sciences Faculty of Sciences Erzurum Technical University 25050 Erzurum, Turkey Dr. V. Pellegrini, Dr. F. Bonaccorso BeDimensional Spa via Albisola 121, 16163 Genova, Italy 1908427 (1 of 10) © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.afm-journal.de near-infrared (NIR) light.[5,10] Several groups reported n-type FET based on single layer and thin InSe crystals with values of electron mobility of µe ≈ 103 cm2 V−1 s−1.[11,12] Photodetectors with a photoresponsivity Rph ≈ 5 × 107 AW−1 in single flake devices,[8,13–18] surpassing TMDs[19,20] and rivalling other high mobility (≥102 cm2 V−1 s−1) direct bandgap 2D semiconductors such as black phosphorus (BP)[21,22] have also been demonstrated. Furthermore, a lower enthalpy of oxygen chemisorption on monolayers of InSe compared to single layer BP makes it more environmentally stable compared to the latter.[23] The high responsivity of β-InSe photodetectors has been explained by the direct bandgap of thick (>5nm) flakes, which results in efficient light absorption in the material.[24] Extrinsic factors due to the photodetector design (e.g., short channel length) may raise the responsivity as well.[18,25] Moreover, the presence of photoconductive gain mechanisms in β-InSe also contributes to the high responsivity, multiplying the number of electrons flowing in the device per absorbed photon.[26] In particular, the presence of traps that reduce the mobility of minority carriers (holes) in the material, results in the circulation of photogenerated electrons multiple times through the device before recombination.[19] Unfortunately, the increased responsivity gain comes at the expense of a slower response time of the photodetector,[27] which might be problematic for applications that require a high bandwidth, such as imaging[28] or optical communication.[29] Furthermore, trap-assisted recombination results in sublinear dependence of the responsivity as a function of the light intensity, limiting the sensitivity at high optical power and complicating the readout of the photodetector signal.[30] Therefore, the study of the charge trapping and recombination mechanisms in InSe could provide insights for the optimization of the semiconducting material and the device design for different application requirements. Although promising figures of merit, FoM, (µe, Rph) have been reported for InSe devices, the aforementioned results were obtained using single-flake devices, produced by low-throughput methods of fabrication, such as mechanical exfoliation[14] of bulk crystals followed by manual assembly[31] of heterostructures. The latter requires the identification of suitable flakes and a precise alignment of the electrodes by electron beam lithography (EBL).[18] Thus, to enable the adoption of this material in consumer electronics, scalable and sustainable processes for both material production and device fabrication are needed. To this end we recall that chemical vapor deposition of InSe films with mostly monolayer thickness was achieved recently over areas of 1 cm2, enabling the fabrication of n-type FETs with electron mobility up to ≈30 cm2 V−1 s−1.[32–34] Wafer scale growth of InSe films by pulsed laser deposition has also been demonstrated, producing films with an electron mobility of 10 cm2 V−1 s−1 and a response of 27 A W−1 in phototransistor devices.[35] These techniques require a fine tuning of the growth parameters (e.g., temperature, growth duration),[36] expensive equipment,[37] and high temperature (>600 °C)[32,35] conditions, which increase the energy requirements for the device fabrication. Therefore, they still do not represent the solution for consumer electronics application. Liquid phase exfoliation (LPE)[38–40] is another process that was successfully exploited to produce few layer InSe crystals, offering advantages over the aforementioned methods in terms Adv. Funct. Mater. 2020, 30, 1908427 of costs and scalability.[41] Although, the compatibility of the inks with high-throughput printing processes and the level of performance of InSe printed devices still needs to be assessed, LPE might represent a viable approach to exploit InSe fewlayers for optoelectronic applications. We remark that LPE allows the exfoliation of layered materials in a liquid medium in ambient conditions,[42] applying an external stimulus such as ultra-sonication,[42,43] high-shear mixing,[44,45] ball milling,[46,47] wet jet milling,[48] and microfluidization.[49] For the efficient exfoliation of layered materials, the solvent should prevent restacking and aggregation of the exfoliated layers by satisfying specific surface energy requirements for the considered material, which are summarized by the solvent surface tension and the Hansen solubility parameters.[50,51] Usually, the dispersions produced by LPE have a heterogeneous composition of exfoliated flakes both in lateral size and thickness,[43] which can be fractioned, for example, by using sedimentation based separation (SBS).[52–55] The as-produced flakes are mostly basal plane defect-free and unfunctionalized.[41,43,53–55] In addition, LPE is the ideal tool for the formulation of inks[50,53,56] and pastes,[57,58] as well as for the integration of the exfoliated flakes in composites,[59] and coatings.[60,61] This makes dispersions of 2D materials appealing for the realization of fully printed devices.[50,53,56] From the production point of view, the choice of the solvent is a critical issue of the LPE process. In fact, common solvents used for the formulation of 2D material based inks (e.g., N-methyl-2-pyrrolidone—NMP, dimethylformamide—DMF, N-cyclohexyl-2-pyrrolidone—CHP), present serious health hazards (e.g., NMP, DMF, CHP, health code ≥ 2 NFPA704).[62] Furthermore, the low vapor pressure (Pv ≈ 0.05 kPa)[63] and high boiling point (Tb ≈ 200 °C)[64] of the aforementioned solvents result in surface contamination of the exfoliated flakes with an insulating organic layer, which is detrimental for the electronic conduction of the exfoliated materials.[18,65,66] Recently, LPE of 2D materials in low boiling point solvents such as acetone,[67] alcohols,[50] and water/alcohol co-solvent systems[18,50,68–70] has been investigated as a strategy to reduce the use of toxic chemicals, minimizing the contamination by solvent residuals in films of LPE flakes. The LPE exfoliation of InSe has been achieved by ultra-sonication by using isopropyl alcohol (IPA)[69] and water-ethanol (H2O/EtOH) mixtures,[18] allowing an effective removal of the solvent by heating at moderate temperatures (e.g., ≈50 °C) thanks to their low boiling point (78.2 °C < Tb < 100 °C).[62] In particular, the Hersam’s group demonstrated that InSe flakes (average thickness of 41 nm) exfoliated by ultra-sonication in deoxygenated H2O/EtOH mixtures exhibit electron mobilities µe ≈ 19 cm2 V−1 s−1 and photoresponsivity Rph ≈ 5 × 107 A W−1 in single flake phototransistors.[18] On the other hand, devices produced with InSe flakes exfoliated in high boiling point solvents (i.e., NMP, DMF) have shown lower performance, compared with the one produced in H2O/EtOH mixtures, due to organic contaminants.[18] The same study reported a responsivity of Rph = 10 AW−1 in devices fabricated from InSe based films obtained by membrane filtration of the dispersions in H2O/EtOH, which resulted in percolating networks of flakes manifesting a deterioration of the photodetector performance compared to the single flake.[18] This phenomenon is a 1908427 (2 of 10) © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.afm-journal.de consequence of the high contact resistance between flakes compared to the intrinsic resistance of the flakes themselves and is exacerbated in the case of percolating films due to electrical conduction occurring through an increased number of interflake boundaries. Although these results highlight the potential of LPE InSe based optoelectronics from a fundamental point of view, the feasibility of scalable and repeatable processes used in industrial production has not yet been investigated. In this work, we produce β-InSe inks by ultrasound-assisted LPE in IPA and use spray coating deposition of the inks on silicon substrates to fabricate highly responsive photodetectors. Spray coating is an industrially compatible process that can be optimized to produce highly uniform films of electronically active materials and has already been applied to the fabrication of perovskite based solar cells.[71,72] Our results show that the photodetectors exhibit high responsivities to light in a broad spectral range (450–900 nm), with a maximum responsivity Rph = 274 AW−1 at 0.53 mW cm−2 irradiance and a rise time of ≈15 ms. Our photodetector outperforms, in term of responsivity, previous devices based on percolating networks of solution processed 2D flakes[18] by more than one order of magnitude. By studying the optical power dependence of the responsivity and the time-dependent profiles of the photocurrent under light modulation, we conclude that the recombination rate of the photocarriers is a cubic function of the concentration of charge carrier density. We suggest that the third order rate dependence could be related to a recombination mechanism assisted by doubly charged Se vacancies. 2. Results and Discussion 2.1. Material Production and Characterization The synthesis of β-InSe monocrystals was performed with the Bridgman–Stockbarger method.[73] InSe can exist in three crystal polymorphs, β, γ, and ε, that differ only in the layers stacking.[3,74,75] The crystal structure obtained by this method has the β form, as determined by X-ray diffraction (XRD).[73] Based on the study of the LPE of β-InSe in IPA reported by Petroni et al.,[69] we performed sonication and ultracentrifugation (see Section 4) in order to achieve a good compromise between ink stability, the lateral size, and thickness of the flakes. The concentration of the exfoliated β-InSe flakes was evaluated by means of optical extinction measurements. The concentration of the obtained dispersion is 0.11 g L−1, as estimated by the Beer–Lambert law αc = A l (1) A is the absorbance per length, c is the concentration l and α is the extinction coefficient with α ≈ 580 L g−1 m−1 at 600 nm.[69] As reported in ref. [69], a typical extinction spectrum of the β-InSe dispersion (Figure S4, Supporting Information) exhibits features at ≈275 nm and at ≈360 nm, which correspond to direct electronic transitions between valence and conduction band.[76] In order to confirm the production of where Adv. Funct. Mater. 2020, 30, 1908427 few layer β-InSe flakes with no defects, the efficacy of the exfoliation of the crystal was evaluated by morphological, chemical, and structural analyses. Raman spectroscopy measurements, shown in Figure 1a, were performed in order to demonstrate the absence of other chemical species, (e.g., In2Se3, In2O3)[75,77] that could have been formed during the exfoliation process or subsequent exposure to atmosphere. β-InSe has six Raman active vibrational modes, which are assigned to the peaks in the spectra in Figure 1a[74,77] in both the bulk and exfoliated β-InSe samples. The data confirms the absence of polymorph crystals. Peaks related to other In and Se compounds or oxidized phases are not present, suggesting that the exfoliated flakes keep their crystalline integrity.[75,77] These conclusions are also supported by X-ray photoelectron spectroscopy (XPS) spectra of the In 3d and Se 3d regions (Figure S5, Supporting Information), showing only a 0.2 eV broadening of the peaks in the exfoliated material, which is indicative of a slight increase in defects and oxide species compared to the bulk, and the XRD patterns shown in Figure 1b. These data are in agreement with the diffraction of β-InSe (ICDD 98-018-5172). The XRD patterns demonstrate, both for bulk and for exfoliated samples, the occurrence of a hexagonal β-InSe structure with lattice parameters of a = b = 4.005 ± 0.004 Å and c = 16.660 ± 0.004 Å.[69] Moreover, the β-InSe bulk crystals exhibit only reflections belonging to the (001) family, indicating strong texture of crystalline flakes along the c axis. In the exfoliated β-InSe flakes, we notice also reflections from other orientations (e.g., 010, 110, and 011), due to disordered arrangement of the nanocrystals, although a strong preferential orientation of the (001) plane is still present. Figure 1c,d reports representative transmission electron microscopy (TEM) and atomic force microscopy (AFM) images of the exfoliated β-InSe flakes, respectively. Statistical analysis using a log-normal distribution[78] (Figure 1e) indicates that the exfoliated β-InSe flakes have a typical lateral dimension (mode) of ≈113 nm (σ = 0.84), corresponding to an average surface area of ≈6.1 × 10−3 µm2, and a thickness of ≈4 nm (σ = 0.54) as measured by AFM in Figure 1f. Therefore, based on the thickness of monolayer β-InSe (≈0.9 nm),[79,80] flakes with single/few layers are effectively produced. 2.2. Phototransistor Fabrication and Characterization We investigated the application of β-InSe based inks in printed photodetectors using a spray coating deposition method. The inks exhibit low viscosity (η25° ≈ 3.2 mPa × s at 25 °C), with Newtonian behavior at shear rates between 0.2 and 300 s−1 (Figure S1, Supporting Information), which makes them suitable for spray coating deposition.[81] An interdigitated gold electrode array was fabricated using EBL and lift-off (see Section 4) on highly p-doped (boron) silicon (100) with 100 nm thermal oxide (SiO2) on the surface. The interdigitated design allows to achieve a low electrical resistivity, by reducing the gap between electrodes L and increasing the total width of the conduction channel W, in a more compact format compared to linear electrodes, by using a parallel array of finger electrodes. Additionally, the reduction of L leads to a faster response of the photodetector[82] by reducing the transit time of the carriers. The electrodes used in our devices consist of 65 interdigitated fingers with an overlap of 40 µm and a gap L = 1 µm, which 1908427 (3 of 10) © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.afm-journal.de Figure 1. Morphological, chemical, and structural characterization of β-InSe flakes. a) Raman spectra for bulk β-InSe (blue trace) and exfoliated β-InSe (red trace) samples. b) XRD diffractogram for bulk β-InSe (blue trace) and exfoliated β-InSe (red trace) samples. c) Representative TEM image of an isolated β-InSe flake. d) Representative AFM image of an isolated β-InSe flake. Height profile (solid white line) of the indicated section (white dashed line) is also shown. e) Lateral size statistical analysis for β-InSe flake dispersion. f) Statistical analysis of the thickness of β-InSe flakes in the dispersion. results in an overall channel width W = 5160 µm, providing an active area of W × L = 5760 µm2 for photodetection. During the same lithographic process, several Au electrode arrays were Adv. Funct. Mater. 2020, 30, 1908427 deposited by e-beam evaporation on the same substrate to exploit the large area deposition capability of the spray coating process. A volume of ≈30 mL of the as-produced β-InSe ink in 1908427 (4 of 10) © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.afm-journal.de Figure 2. a) Schematic illustration of the β-InSe photodetector. b) Optical microscope image of the complete photodetector and c) magnified detail of the conducting channel region. d) SEM image of the β-InSe film in the conducting channel region of the device. e) Dark source–drain current (ID) versus source–drain voltage (VD), at bottom gate voltages between −40 and 40 V. In the inset, a log–log plot of ID versus VD (VG = 0 V) is depicted, showing the transition from ohmic to space charge limited conduction. f) Source–drain current (ID) versus gate voltage (VG) curve at VD = 2 V, obtained from the data in (e). IPA was deposited by spray-coating onto the patterned substrate, which was heated to 60 °C to favor film drying during the deposition process. The coated sample was placed under vacuum overnight at room temperature and then annealed at 200 °C for 30 min in argon atmosphere to stabilize the deposited film and remove residues of adsorbed solvent and moisture. The film thickness was 515 ± 107 nm, as measured by profilometry (Figure S2, Supporting Information). The basic structure of the β-InSe photodetector is illustrated by the scheme in Figure 2a and optical microscope images in Figure 2b,c. Figure 2d shows the top-view scanning electron microscopy (SEM) image of the β-InSe film deposited on the electrodes, presenting a percolating network of flakes of lateral size <500 nm. Energy-dispersive X-ray (EDX) spectroscopy (Figure S3, Supporting Information) was performed during SEM imaging, confirming the presence of β-InSe flakes in the film. From the EDX analysis on the device, we determined a ratio of 1:0.87 between In and Se, in agreement with previous characterization of the exfoliated β-InSe performed by our group.[69] The elemental composition, within the experimental uncertainty of EDX, indicates the presence of selenium vacancies in the semiconductor film.[69] The signal from oxygen can be explained mostly by the SiO2 layer on the surface of the substrate, although a superficial oxidation of the β-InSe flakes is also possible due to the processing in air. The current–voltage (ID–VD) curves of the β-InSe photodetector were measured at room temperature and under vacuum Adv. Funct. Mater. 2020, 30, 1908427 to avoid the influence of moisture and atmospheric gases. The doped silicon substrate was electrically contacted to apply a gate bias VG. As shown in Figure 2e, the drain-source current (ID) varies with drain voltage VD under different VG (from −40 to 40 V) in dark condition, displaying a gate-tuneable behavior, indicating that the electrical characteristics of β-InSe device can be effectively controlled by electrostatic doping. In the inset to Figure 2e, a quasi-linear regime (slope of the curve r = 1.2) for 0 < VD < 0.5 V suggests ohmic contacts between the gold electrode and β-InSe. At larger bias with 0.5 < VD < 2 V, a quadratic dependence of the drain current is observed, which can be explained by a space-charge limited current (SCLC) regime.[83] SCLC becomes dominant when the injected electron concentration exceeds that of the thermally generated one. Charge-carrier traps within the semiconducting film, located, for example, in the partially oxidized surface of the flakes, are effective in immobilizing most of the injected electrons.[84,85] For shallow traps located at an energy Et below the conduction band edge, the SCLC current density is given by a quadratic relation with respect to the voltage (I D ∝ VD2).[83] Figure 2f shows the ID–VG transfer curves in the dark at a drain bias voltage of 2 V. The device exhibits an ambipolar behavior with a minimum ID current at VG ≈ −10 V and a distinct asymmetry in electron and hole conduction, showing an average mobility of the film (defined in the linear regions of the ID–VG curve) of 3 × 10−5 cm2 V−1 s−1 for the electrons and 1 × 10−5 cm2 V−1 s−1 for the holes. We attribute the decrease in mobility compared to single flake 1908427 (5 of 10) © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.afm-journal.de Figure 3. a) Photocurrent response for varying incident light power on the active area of the device measured at three different wavelengths. The dashed lines represent a power-law fit to the data. The inset plots the photoresponsivity Rph versus the irradiance Ir. b) Spectral responsivity of β-InSe photodetector. The dashed line is a linear fit of the tail of the spectrum; the onset of the photoresponse at ≈900 nm (1.38 eV) is highlighted. The inset plots the response to 0.1 Hz pulsed light (530 nm) c) Detail of photocurrent rise and decay during pulsed (2 Hz) 625 nm light excitation. d) Plot of the time derivative of the photocurrent versus the photocurrent after switching off the light. Inset: Responsivity versus light chopping frequency under illumination at 625 nm. Dashed lines indicate the power laws f−0.5 and f−0.2. values to the presence of traps and the surface oxidation during spray deposition processing in air, which increases the contact resistance between the flakes.[86] The transistor is n-doped at VG = 0 V and, for this reason, no off-state condition can be measured. This phenomenon could be explained by the presence of the defect states in the gap of β-InSe. The n-doping in InSe nanoflakes was also confirmed by the position of the Fermi level compared to the valence and conduction bands, as determined through ultraviolet photoelectron spectroscopy (UPS) (Figure S6, Supporting Information). In order to reveal the photodetection behavior of the device, the electrical characteristics were measured under the exposure to light of different wavelength and intensity. The photodetector response is measured as photocurrent (Iph), corresponding to the difference between the current measured (at VD = 2 V, VG = 0 V) under light and in the dark, for light intensities (Ir) between 0.5 and 7.5 mW cm−2. Figure 3a reports the photocurrent as a function of the light power on the device, using three different LEDs with emission peaks at 455, 530, and 625 nm (FWHM = 17, 42, 20 nm, respectively). The dependence on light power is sublinear; therefore, the photoresponsivity (Rph, inset of Figure 3a), defined as R ph = I ph Ir ⋅ S Adv. Funct. Mater. 2020, 30, 1908427 (2) (in which Ir is the irradiance, S is the active area of the device), is decreasing as the power increases. The maximum measured responsivity at 455 nm is ≈274 AW−1, at ≈0.53 mW cm−2 irradiance. The device maintained its functionality even in ambient conditions and after 1 year (Figure S7, Supporting Information). To assess the minimum detectable light intensity, we calculated the detectivity, defined as[87] D* = S NEP (3) Si is the noise equivalent power and Si is Rph the spectral density of the noise current. We obtained a maximum value (corresponding to the maximum responsivity) of NEP ≈1.31 × 10−15 W Hz−0.5 and D* ≈ 5.49 × 1012 Jones. These values are obtained estimating Si by the sum of two noise mechanisms: the thermal noise Sc = (4kT/Rdark)0.5, calculated considering the dark resistance Rdark of the device, and the shot noise, Ss = (2eIdark)0.5, from the dark current Idark. These FoM values compare favorably with those reported in literature.[88–91] The responsivity is at least one order of magnitude higher than those of photodetectors based on solution processed 2D crystals, and InSe in particular.[18] There are several factors that can cause the increased responsivity of our device. The short length of the channel (L) increases the In which NEP = 1908427 (6 of 10) © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.afm-journal.de photoresponsivity of the device, since Rph is proportional to L−2.[25] Moreover, the highly uniform coverage of β-InSe on the Si/SiO2 substrate over the active area could promote the extraction efficiency of the photogenerated carriers as already suggested in literature for perovskite-,[92] thin-film CdTe[93] and graphene based devices.[94] The spectral response of the device is shown in Figure 3b from the visible to the NIR range, measured with mechanically chopped light at 23 Hz (see Section 4) at bias voltages VD = 2 V and gate voltage VG = 0 V. The responsivity is high at short wavelength (≈400 nm) and decreases towards longer wavelength with a pronounced drop at ≈800 nm. If the wavelength dependence is dominated by the absorption coefficient, it is possible to estimate the bandgap from the intercept with the wavelength axis of a linear fit of the spectral responsivity in the region near the photoresponse threshold wavelength (Figure 3b). The resulting value of ≈900 ± 65 nm corresponds to a bandgap of ≈1.4 ± 0.1 eV, which is in the range between the values reported for single layer and bulk β-InSe (2.11–1.26 eV respectively), confirming that the film is composed by few layer flakes. These results are also supported by the observation of a broad photoluminescence (PL) peak at 959 ± 50 nm (Figure S8, Supporting Information). Finally, in Figures 3b (inset) and 3c we report the time response of the device, obtained by measuring the current during pulsed light excitation. Excellent long-term stability is observed with a fast switching and reproducible transition from “ON” to “OFF” conditions. The measured rise (fall) time (Figure 3c), defined as the time to pass from 10% (90%) to 90% (10%) of the maximum photocurrent, is τR = 15.1 ms (τF = 63.7 ms), while other photodetectors made with films of LPE flakes of β-InSe reported response times of the order of seconds.[10,18] In the inset to Figure 3d, we report the dependence of the photocurrent amplitude on the modulation frequency f. The amplitude decreases with the increase of the frequency, and the signal is measurable up to 1 kHz; the functional dependence of the frequency does not follow a simple low-pass behavior but exhibits richer features with complex power-law trends that follow a f−0.5 and f−0.2 dependence at low and high modulation frequency respectively, as shown by dashed lines in the panel. Such behavior could indicate an energy distribution of trap states in the material. The time decay of photocurrent in Figure 3c can be investigated more in detail to understand the mechanism of charge recombination in our device. In general, the time derivative of the photoexcited charge density n, after turning off the light, is a function of n itself. dn = g (n ) dt (4) Assuming that the photocurrent is proportional to n, the function g can be determined from the time dependence of Iph during the decay measurement. In Figure 3d we plot the photocurrent derivative (dIph(t)/dt) versus Iph after light was switched off. A third-order polynomial yields an excellent fit to the data (dashed line in Figure 3d), suggesting that the differential equation controlling generation–recombination under light is dn = F − Rr n 3 dt Adv. Funct. Mater. 2020, 30, 1908427 (5) Here, F is the generation rate due to incoming light, and Rr is a constant controlling the recombination rate. This dependence is further confirmed by the photocurrent vs light power dependence (Figure 3a). Assuming that steady state is reached for these measurements (dn/dt = 0), by solving Equation (5) we find a dependence as Iph ∝ n ∝ Fϑ, with ϑ = 1/3. In fact, the fits shown in Figure 3a yield ϑ625 = 0.324 ± 0.003, ϑ530 = 0.318 ± 0.004, ϑ455 = 0.293 ± 0.013, confirming this prediction. The observed power-law dependence is related to the dynamics of the traps and recombination centers that enable the photoconduction in nanostructured materials. A nonlinear dependence of the photocurrent on the light intensity can arise from the distribution of traps and recombination centers within the band gap[95,96] and the saturation of these states under strong light excitation,[13,18,97–99] or even from the presence of a space charge region.[100,101] The particular case of the thirdorder equation, as suggested in ref. [30], can be related to the recombination assisted by doubly charged Se vacancies, which have been observed in our sample by EDX analysis and by previous studies on β-InSe.[69,84,85] We remark that doubly charged vacancies in CdS have been related to more complex processes involving not only charge excitation, but also charge-lattice coupling that lead to a thermally activated recombination behavior,[30] suggesting that further studies of these properties can lead to interesting physics of the material. 3. Conclusion We demonstrated that spray-coated liquid phase exfoliated β-InSe photodetectors have good photosensitive responsivity to visible and NIR (<900 nm) light and can be used as semiconductor channel material in phototransistors, obtaining a high-throughput and low-cost method to implement βInSe photodetectors with high sensitivity (maximum D* ≈5.49 1012 Jones, Rph ≈ 274 A W−1 at 455 nm illumination) and fast response time (τR ≈ 15 ms, τF 64 ms). These FoM represent an improvement of over one order of magnitude with respect to reported photodetectors based on percolating networks of solution processed 2D flakes[18] and are competitive with the values reported for transition metal dichalcogenide single flake devices.[91] The use of spray coating and environmentally friendly solvents in the fabrication process makes this material an interesting alternative for photodetectors in the visible and NIR spectral range, also for applications on flexible substrates. The gate tuneable transport and the cubic photo-charge recombination law behavior observed in β-InSe nanoflakes films prompt further fundamental studies of this mesoscopic system. 4. Experimental Section Exfoliation of Bulk β-InSe: β-InSe single crystals were grown by the modified Bridgman–Stockbarger method, as described elsewhere.[73] A quantity of 40 mg of β-InSe was exfoliated in 20 mL IPA (ACS Reagent, ≥99.8%, Sigma-Aldrich) through LPE[42] in a sonic bath (6 h, 25–35 °C, Branson 5800 cleaner, Branson Ultrasonics), followed by SBS[102] (1000 × g for 30 min at 15 °C, Beckman Coulter Optima XE-90) and collection of the supernatant (≈80%), as described in ref. [69]. 1908427 (7 of 10) © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.afm-journal.de Material Characterization: The extinction spectra of the exfoliated InSe flake dispersions in IPA diluted 1:10 was measured using a Cary Varian 5000 UV–vis, subtracting the extinction of pure solvent. TEM analysis was carried out on a JEOL JEM 1400Plus microscope (120 kV), with a LaB6 source, a Gatan CCD camera Orius 830. Samples were prepared by drop-casting the flake dispersions onto carbon-coated Cu grids and dried overnight in vacuum. The XPS analysis was performed on a Kratos Axis UltraDLD spectrometer at a vacuum better than 10−8 mbar, using a monochromatic Al Kα source operating at 20 mA and 15 kV and collecting photoelectrons from a 300 × 700 µm2 sample area. Wide spectra were acquired at pass energy of 160 eV and energy step of 1 eV, while high-resolution spectra were acquired at pass energy of 10 eV and energy step of 0.1 eV. The samples were prepared by drop-casting the dispersion of InSe flakes onto Au-coated Si chip in N2 atmosphere, while heating the substrate to 60 °C. Bulk InSe crystals were sticked onto conductive carbon tape and cleaved prior analysis. The samples were transferred to the XPS chamber in inert atmosphere. Data analysis was carried out with CasaXPS software (version 2.3.19PR1.0). The energy scale was calibrated by setting the C 1s peak at 284.8 eV. UPS was performed with He I (hν = 21.2 eV) radiation to obtain the workfunction and the position of the valence band maximum of the materials under investigation. The experiments were carried out on the samples after the XPS analysis using the same equipment. A −9.0 V bias was applied to the sample in order to determine the low kinetic energy cut-off. AFM images of the samples were acquired using a Nanowizard III (JPK Instruments, Germany) mounted on an Axio Observer D1 (Carl Zeiss, Germany) inverted optical microscope, using PPP-NCHR cantilevers (Nanosensors, USA). Samples were prepared by dropcasting the exfoliated InSe flake dispersions onto mica sheets (G250-1, Agar Scientific Ltd., Essex, UK). The images (512 × 512 data points) of 5 × 5 µm2, 2.5 × 2.5 µm2, and 500 × 500 nm2 were collected at a scan rate of 0.6 Hz in intermittent contact mode by keeping the set point above 65% of the free oscillation amplitude. Height profiles of ≈100 flakes were analyzed with JPK Data Processing software (JPK Instruments, Germany). Raman spectroscopy measurements were carried out with excitation wavelength of 514.5 nm and incident power of 1 mW, using a Renishaw micro-Raman InVia system equipped with a 100× objective. The samples were prepared by drop-casting InSe flake dispersions on Si/SiO2 substrates and analyzed after drying in vacuum by collecting 20 spectra in different positions. The spectra were fitted with Lorentzian functions. For micro-photoluminescence spectra, a Renishaw inVia was used, equipped with a 50× (0.75 N.A.) objective and laser excitation wavelength of 785 nm. The PL spectra were recorded on films deposited on quartz with a time exposure of 10 s and laser power ≈1 mW. The bulk sample was measured with a time exposure of 1 s and laser power of ≈50 µW. The crystal structure was characterized by XRD using a PANalytical Empyrean diffractometer with Cu Kα radiation. The samples were obtained by deposition of InSe dispersions on Si substrates. The viscosity of the InSe ink in IPA was measured with a Discovery HR-2 Hybrid Rheometer (TA instruments), using a double-wall concentric cylinders geometry (inner diameter of 32 mm and outer diameter of 35 mm), designed for low-viscosity fluids. The temperatures of the inks were set and maintained at 25 °C throughout all the measurements. The SEM/EDX measurements on the devices were performed using a Helios Nanolab 600 (FEI Company) combined with an X-Max detector and INCA s system (Oxford Instruments) for the EDX spectra acquisition and analysis. For SEM measurements an accelerating voltage of 10 kV and beam current of 0.2 nA were used, while for EDX measurement these values were set to 10 kV and 0.8 nA. The sample was imaged as it was, without any conductive coating applied to the surface. Device Fabrication: A set of nine interdigitated source/drain electrodes for FETs were patterned on a highly p-doped Si chip covered with a 100 nm layer of thermal oxide. Each source/drain electrode pair consisted of 65 pairs of interdigitated fingers with an overlap of 40 µm and a gap L = 1 µm, which resulted in an overall channel width W = 5160 µm and an active area of W × L = 5160 µm2 for photodetection. Adv. Funct. Mater. 2020, 30, 1908427 Source (S) and drain (D) contacts were patterned by EBL with poly(methyl methacrylate) (PMMA) resist, metal e-beam evaporation (5 nm Ti/30 nm Au) using magnets to not expose the PMMA, followed by lift-off. The device was completed by spraying the entire surface of the chip with ≈30 mL of the β-InSe ink in IPA, while heating the substrate to 60 °C in air. The coated sample was placed under vacuum overnight at room temperature and then annealed at 200 °C for 30 min in argon. Device Characterization: Electrical characterization was performed at room temperature in vacuum, using a shielded probe station equipped with a Keithley 2612 source meter with two channels to control both the drain and the gate bias potential with respect to the source terminal of the device. To keep its stability and reduce the effect from surface adsorbates, the device was measured under vacuum (≈10−5 mbar). The backside of the doped silicon substrate was electrically contacted to a gold pad with silver paint to work as the gate electrode. The measurement procedure was controlled by a PC using a program witten in LabVIEW (National Instruments). For the photoresponse measurements, LEDs with nominal wavelength of 455, 530, and 625 nm (M455L3, M530L3, M625L3 Thorlabs) were used. Light was focused on the device with a collimating lens. The LED output power was controlled by the power supply (LEDD1B Thorlabs). Before measuring the device, the irradiance at the center of the focused light spot was determined with the following procedure: the light was focused on a pinhole (⌀ = 3.37 mm) and the power of the transmitted light was measured with a silicon photodiode (S120VC, Thorlabs) connected to a power meter (PM100D, Thorlabs), allowing the calculation of the irradiance at the center of the spot by dividing the power by the area of the pinhole. A 10% loss due to the reflection of the optical window of the vacuum probe station was taken into account for the calculation. Spectral measurements were performed with a Xe lamp coupled to a Spectral Products CM110 monochromator. The light was modulated by a mechanical chopper at 23 Hz and the AC current was measured with a Signal Recovery DSP 7265 lock-in after amplification by a DL1211 current amplifier. The same setup was used for frequency modulation measurements. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements N.C. and M.S. contributed equally to this work. N.C. gratefully acknowledges the financial support of a Ph.D. scholarship from Region Sardinia (P.O.R. Sardegna, European Social Fund 2007–2013—Axis IV Human Resources, Line of Activity I.3.1). The authors thank the Clean Room Facility of the Italian Institute of Technology for support with device fabrication and Dr F. De Angelis (Plasmon Nanotechnologies group) for the access to the Raman equipment. Conflict of Interest The authors declare no conflict of interest. Keywords 2D semiconductors, field effect transistors, indium selenide, liquid phase exfoliation, photodetectors, solution processed, spray coating 1908427 (8 of 10) Received: October 12, 2019 Revised: January 5, 2020 Published online: February 14, 2020 © 2020 WILEY-VCH Verlag GmbH & Co. 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