Synergistic Effect of Hybrid Multilayer In2Se3 and Nanodiamonds for

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Synergistic Effect of Hybrid Multilayer In2Se3 and Nanodiamonds for Highly Sensitive Photodetectors Zhaoqiang Zheng, Jiandong Yao, Jun Xiao, and Guowei Yang* State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, School of Materials Science & Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, P. R. China S Supporting Information *

ABSTRACT: Layered materials have rapidly established themselves as intriguing building blocks for next-generation photodetection platforms in view of their exotic electronic and optical attributes. However, both relatively low mobility and heavier electron effective mass limit layered materials for highperformance applications. Herein, we employed nanodiamonds (NDs) to promote the performance of multilayer In2Se3 photodetectors for the first time. This hybrid NDs−In2Se3 photodetector showed a tremendous promotion of photodetection performance in comparison to pristine In2Se3 ones. This hybrid devices exhibited remarkable detectivity (5.12 × 1012 jones), fast response speed (less than 16.6 ms), and decent current on/off ratio (∼2285) simultaneously. These parameters are superior to most reported layered materials based photodetectors and even comparable to the state-of-the-art commercial photodetectors. Meanwhile, we attributed this excellent performance to the synergistic effect between NDs and the In2Se3. They can greatly enhance the broad spectrum absorption and promote the injection of photoexcited carrier in NDs to In2Se3. These results actually open up a new scenario for designing and fabricating innovative optoelectronic systems. KEYWORDS: layered materials, In2Se3, nanodiamonds, synergistic effect, photodetectors

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INTRODUCTION Low-cost high-sensitivity photodetector is at the heart of a multitude of technologies that affect our daily lives. Applications spanning the fields of video imaging, optical communication, remote sensing, environment monitoring, security check and industrial processing control, and photodetector have reached a high level of maturity attributed to the development of high-performance materials and large-scale production and integration technologies.1−3 Over the past decade, layered materials, such as metal dichalcogenides4,5 (MoS2, WSe2), III−VI semiconductors6,7 (GaSe and In2Se3), and elemental semiconductors8 (black phosphorus), offer the potential to revolutionize the photon-detection industry toward high-sensitivity detectors, in view of their exotic electronic and optical attributes. As a representative, MoS2 is the most widely investigated layered material. Highly sensitive MoS2 based photodetectors have been reported to cover the broad spectrum from the UV−vis up to the near-infrared (NIR).9−11 However, the relatively low mobility and heavier electron effective mass limit these devices for high-performance application.12 These photodetectors either demonstrate a responsivity up to 880 A/W with long response time of 4 s, or exhibit a low responsivity of 7.5 mA/W with fast response of 50 ms, or possess an extremely high current on/off ratio reaches 108 with a poor responsivity less than 1 A/W.9,10,13 It is thus difficult to balance high responsivity, fast response speed, and superior current on/off ratio simultaneously. Additionally, for aforementioned photodetectors, an additional large gate © XXXX American Chemical Society

bias (Vg) is essential for obtaining high sensitivity. Such large Vg may not only induce a leakage current between the source and gate and cause power dissipation but will also restrict the degree of freedom in external circuit designing, which would hinder their practical application. Besides, for the active layer, remarkably, direct bandgap materials hold high absorption coefficient and allow generate electron−hole pair efficiently under photoexcitation. Therefore, a layered material with a natural direct bandgap could be an optimum candidate for photodetector applications.14−16 Keeping this fundamental scenario in mind, we found that In2Se3 was the desirable candidate due to the direct bandgap in bulk,17 while most layered materials (like MoS2) just showed a direct bandgap in monolayer form.9,18−20 Carbon nanomaterials always occupy an important position in the development of modern science and technology thanks to their unique photoelectric properties.21 Nondiamond carbons dots (CDs), especially graphene quantum dots (GQDs), are ideal building blocks for nanoscaled-hybrid materials and have been applied in light-absorbing materials in various photodetectors and photovoltaic devices.22−24 The combination of CDs and layered materials always demonstrated impressive optical response. In these devices, CDs have been used as hole acceptor to potent suppression of recombination Received: June 1, 2016 Accepted: July 20, 2016

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DOI: 10.1021/acsami.6b06531 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a, b) TEM and high-resolution TEM images of NDs. (c) Raman patterns of graphite powders and NDs. (d) XRD diffraction patterns and (e) SEM image of the In2Se3 film. (f) Height profiles along the edge of a scratch in In2Se3 film. Inset shows the corresponding AFM image. The thickness of the In2Se3 sample is deduced to be ∼18.4 nm. (g) AFM surface topography image of the whole device. The device is composed of multilayer In2Se3 film with Cr/Au contact.

by laser-irradiating the ethanol suspension of graphite, while multilayer In2Se3 is deposited through the pulsed-laser deposition (PLD) approach. Then we systematically investigate the optoelectronic properties of the constructed photodetector. Interestingly, this device exhibits a remarkable detectivity of 5.12 × 1012 jones, along with photoresponsivity of 33.6 A/W and external quantum efficiency of 7841%, plus an ultrafast response/recovery time less than 16.6/24.9 ms for the rise/fall. Importantly, it is worth to note that the relatively high current on/off ratio of ∼2285 is achieved using Au contact, attributing to the formation of Schottky barrier to depress the dark current. These parameters balance high detectivity, fast response speed, and superior current on/off ratio simultaneously. Besides, they outperform the majority of photodetectors based on layered materials (details are given in Table S1 of the Supporting Information). Meanwhile, we attribute the high sensitivity of this NDs−In2Se3 hybrid device to the synergistic effect of the appearance of abundant surface functional groups and small sp2-hybridized C fraction vests NDs and the suitable band structures of NDs. They may greatly enhance broad spectrum absorption and promote the injection of photoexcited carrier in NDs to In2Se3. To the best of our knowledge, this is the first

activity, and CDs can also act as nanoscaled light sources to provide additional energy that could be absorbed by the adjacent semiconductor owing to their up- or down-converted photoluminescence property. On the other hand, the other fascinating carbon nanomaterials, nanodiamonds (NDs), which is primarily composed of sp3-hybridized carbon (C) atoms, have been wielded in photocatalytic hydrogen evolution reaction (HER)25,26 but scarcely applied in photodetection. Actually, NDs have the lowest toxicity among all kinds of carbon nanomaterials, which endows them a greater potential in practical applications.27 Besides, the high room temperature thermal conductivity, chemical stability, and resistance to photobleaching ensure NDs keep a long-term stability during the photodetection.28,29 In addition, the unrivaled carrier mobility of NDs could be in favor of the photogenerated carriers transfer to the electrodes.30 Keeping these issues in mind, we believe that NDs can overcome the defects mentioned above of layered material based photodetectors. In this contribution, we chose NDs−In2Se3 hybrids as active layer and then deposit Au electrodes for the fabrication of a two-terminal photodetector. For our study, NDs are achieved B


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dropped on the channel between parallel electrodes and dried at 60 °C for several hours to vaporize ethanol. The electrical measurements were carried out on a Lakeshore probe station equipped with a semiconductor characterization system (Keithley 4200) at room temperature under ambient conditions. For photodetection, laser-driven light sources (Viasho) were employed to provide incident light with wavelengths ranging from ultraviolet to the near-infrared. The time-dependent photoresponses were recorded after switching on/off the illumination.

time to improve the semiconductor photodetection performance by NDs. This finding may open up a new scenario for the development of innovative optoelectronic systems.

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EXPERIMENTAL SECTION

NDs Preparation and Characterization. Laser ablation in liquid (LAL) was exploited to prepare NDs, and the process parameters are similar to those used in our previous work.25,31,32 In our study, about 4 mg of raw graphite powders (Alfa Aesar) was put into a 10 mL glass bottle filled with absolute ethanol (Guangzhou chemical reagent). After ultrasonic oscillating for 15 min to form a suspension, a second harmonic produced by a Q-switched Nd:YAG laser device with a wavelength of 532 nm, pulse width of 10 ns, repeating frequency of 10 Hz, and energy of 180 mJ was focused into the middle of the suspension by a lens with 1 mm spot size. During the laser irradiation, the suspension was kept stirring with a magnetic stirrer, and this process lasted for about 3 h until no obvious black solid powders were suspending. After ablation, the suspension was centrifuged at 6000 rpm for 5 min to remove the residual graphite powders. Finally, the above NDs ethanol solution was sealed and stored in the dark for 3 days before use. The morphology of NDs was observed by a transmission electron microscopy (TEM, FEI Tecnai G2 F30). The Raman spectra were collected through Renishaw InVia spectrometer using 514 nm Ar ion laser for excitation to distinguish the graphite powders and NDs. Highresolution X-ray photoelectron spectroscopy (XPS, Escalab 250, Thermo-VG Scientific) with a monochromatic Al Kα source was used to measure the binding energies of C 1s of the NDs with respect to the position of the Au 4f at 84.0 eV. XPS spectra of valence band (VB) were also recorded. Fourier transform infrared (FTIR) spectrometer (Nicolet6700, Thermo Scientific) was introduced to determine the functional groups on the surfaces of NDs. UV−vis−NIR diffuse reflectance spectra (DRS) were recorded by a UV−vis−NIR spectrophotometer (Lambda 950, PerkinElmer) using BaSO4 as the reference. In2Se3 Film Preparation and Characterization. High-quality In2Se3 film was synthesized by the PLD method, which has been exploited to deposit other layered materials, such as black phosphorus,33 GaSe,7 TMDs,34,35 Bi2Te3,36 etc. First, silicon substrates with a covering oxide 300 nm thickness were washed by ultrasonic cleaning in alcohol and deionized water for 20 min to obtain an intact surface. After drying the substrates in a fume hood, they were mounted immediately on a rotating holder, which is located at a distance of 7 cm away from and parallel to the solid targets of In2Se3 (99.99%) in the deposition chamber. Then, after evacuating the deposition chamber to a base pressure less than 2 × 10−4 Pa, the evacuating process is followed by hearting the substrates to 360 °C. Sequentially, high purity argon gas was introduced into the system as the background gas at the rate of 50 sccm, and the background pressure was maintained at 20 Pa. Thereupon, a pulsed laser beam produced by a KrF excimer laser with a wavelength of 248 nm, pulse duration of 20 ns, and repetition rate of 4 Hz was focused to ablate the target. The total pulse number is 3000, and the operated energy of each pulse was constant at 100 mJ. It should be noted that a shadow mask was used to cover the Si/SiO2 substrate for patterning the In2Se3 film. The surface morphologies and thickness profile of the In2Se3 film were acquired by a scanning electron microscope (SEM, FEI Quanta 400F) and an atomic force microscope (AFM, Bruker Dimension Fastscan). X-ray diffraction (XRD, Rigaku D-MAX 2200 VPC) was recorded at a speed of 8° min−1 at room temperature. The operating voltage and current were 40 kV and 26 mA, respectively. Ultraviolet photoemission spectroscopy (UPS) measurement was carried out using helium I with photon energy of 21.22 eV to determine the Fermi energy (Ef) of the In2Se3 film. Device Fabrication and Characterization. In the fabrication process, the electrical contacts were patterned on the In2Se3 film by using a standard photolithography process. Cr (5 nm) and Au (100 nm) electrodes were deposited subsequently by thermal evaporation and formed a channel of L/W = 2 mm/20 μm. The NDs solution was

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RESULTS AND DISCUSSION Morphology and Structure of the Hybrid NDs−In2Se3 Photodetector. As illustrated in Figure 1a, we can find that there are a lot of disperse and uniform nanoparticles with sizes ranging from 1.5 to 3 nm. The size distribution histogram and corresponding Gaussian fitting curve as shown in Figure S1 reveal that the as-synthesized nanoparticles have an average diameter of 2.29 nm. High-resolution TEM image in Figure 1b reveals that the interplanar spacing is 0.206 nm, corresponding to the (111) crystallographic plane of cubic diamond.25 To further assess the crystal structure, Raman patterns of the graphite powders and NDs are recorded in Figure 1c. In detail, graphite powders present two features at 1349 and 1591 cm−1; they are attributed to the D-band and G-band of sp2-hybridized C, respectively. For NDs, the well-defined peak at 1321 cm−1 is a typical signal of diamond, which confirm that the NDs have been prepared successfully from graphite powders, while the upshifting G-band peak around 1650 cm−1 can be assigned to the interaction effect between sp2-hybridized C and surface functional groups.32 In addition, the XRD patterns of the graphite powders and NDs are shown in Figure S2, which indicates the phase transformation from graphite (JCPDS 261079) to diamond (JCPDS 43-1104). In detail, the diffraction peaks at 2θ = 26.6° and 2θ = 54.8° of the original graphite powders belong to the (003) and (006) planes, respectively. On the other hand, the peak at 2θ = 43.9° can be indexed as the (111) plane of the cubic diamond. Note that the broad peak around 2θ = 23.6° may originate in some organic groups on the surface of NDs, which will be discussed below. In the following PLD process, the In2Se3 film was deposited on the Si/SiO2 substrate. The phase purity and structure of the as-prepared film were investigated by XRD. As presented in Figure 1d, all diffraction peaks can be indexed as beta phase In2Se3 (JCPDS 35-1056),37 implying its layered crystallographic structure,17 and the sharp diffraction peaks indicate its high crystalline quality. In addition, the predominant diffraction peaks local at 2θ = 9.36° and 2θ = 18.79° can be indexed as the (003) and (006) planes, respectively, indicating its highly c-axisoriented nature, which is a benefit for carriers in-plane transport.38 From the SEM image in Figure 1e, we can observe continuous polycrystalline morphology. To measure the thickness of the In2Se3 film, an AFM image at the edge of the sample and the corresponding height profiles are shown in Figure 1f. The thickness of the In2Se3 film is deduced to be 18.4 nm, approximately 18 layers.39 Interestingly, as reported in recent analogous works, multilayer layered materials could even be better candidates in electronics/optoelectronics than their monolayer counterparts owing to their weaker negative influences, such as screening effects from interfaces with the substrates or electrodes.40−42 To construct a photodetector, Au electrodes contacts were prepared on the In2Se3 film using the thermal evaporation method. The AFM image of the constructed device is shown in Figure 1g. The channel spacing between two parallel electrodes C


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Figure 2. (a) Schematic diagram of the In2Se3 phototransistor modified with NDs under light illumination. (b) I−V characteristics of the NDs− In2Se3 phototransistor under different illumination power densities. Inset is the higher magnification I−V characteristics under dark and weak illumination. (c) The corresponding logarithmic I−V characteristics. (d) Power intensity dependent photocurrent (blue) and responsivity (R, magenta) under an excitation wavelength of 532 nm at Vds = 5 V. (e) 3D responsivity map of the NDs modified In2Se3 photodetector. (f) Power intensity dependent external quantum efficiency (EQE, blue) and specific detectivity (D*, magenta) at Vds = 5 V.

in Figure 2a. We first measured the current vs bias voltage (I− V) characteristics of the hybrid NDs−In2Se3 devices in dark and under light illumination (532 nm) with various power intensities ranging from 0.025 to 27 mW/cm2, and the results were linear and logarithmic and are presented in Figures 2b and 2c, respectively. The insets in Figure 2b are the higher magnification I−V characteristics under dark and weak illumination. The unsymmetrical I−V curves in the inset of Figure 2b indicate Schottky contacts between the active film and Au electrodes, which should also play an important role in the high performance of our photodetectors. The Schottky barrier located at the contact areas can not only accelerate the

was maintained at 20 μm. The AFM surface topography of the In2Se3 film before and after being coated with NDs is observed in Figure S3. The roughness (RMS) of the pristine In2Se3 film was calculated to be 1.5 nm, indicating a smooth surface. While the film becomes very rough after being coated with NDs, the RMS was increased to 7 nm. Optoelectronic Performance of the In2Se3 and NDs− In2Se3 Photodetectors. Photoresponse measurements are conducted to verify our assumption that NDs can be employed to improve the photodetection performance of the In2Se3 film. The 3D device architecture schematic view of the hybrid NDs− In2Se3 photodetector with laser beam illumination is illustrated D


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Figure 3. (a) Time-resolved switching stability of the two devices response to a pulsed weak illumination (λ = 532 nm, P = 0.025 mW/cm2) under Vds = 5 V. (b) Temporal response of a single response cycle; the time interval is 8.3 ms.

still exhibit significant photocurrent (inset in Figure 2b and Figure S4a) and the current on/off ratio (Ilight/Idark) up to ∼7 (Figure S5). Restricted by the instrument, we have not measured the Rλ at even lower light illumination, but we believe that if a lower incident light intensity (such as to the order of nW or pW/cm−2) is chosen, a higher responsivity can be expected. With this measured Rλ here, as presented in Figure 2f, the EQE of pristine In2Se3 and hybrid NDs−In2Se3 devices can be estimated to be ∼4058% and ∼7814%, respectively, using the relation EQE = hcRλ/λe, where h is Planck’s constant, c is the light velocity, λ is the excitation wavelength, and e is the electronic charge.46 Furthermore, the photoresponsivity does not only depend on the measurement conditions such as light intensity but also strongly dependent on the device geometry.49 The most important figure of merit for a photodetector is the specific detectivity (D*),50 which allows direct comparison between photodetectors with different geometry. The specific detectivity is inversely proportional to the noise equivalent power (NEP), which is given by D* = S1/2/NEP. Considering a dominating shot noise in the dark state current Idark, we have NEP = (2eIdark)1/2/Rλ and D* = RλS1/2/(2eIdark)1/2. With Idark = 53.75 nA at Vds = 5 V for the NDs−In2Se3 device (Figure 2c), the NEP is thus determined to be 3.91 × 10−15 W, giving rise to a D* of 5.12 × 1012 jones (Figure 2f), measured with a 532 nm laser under P = 25 μW/cm2. This performance value is superior to most of the previously reported values for photodetectors based on layered materials, such as solution-processed Bi2S3 (1011 jones),51 few-layered HfS2 (1.3 × 1010 jones),12 and thin crystal SnS2 (2 × 109 jones),49 while this D* value is also comparable to the state-of-the-art commercial Si (3 × 1012 jones) and Ge (3 × 1011 jones) photodetectors.50 Next, considering that the maximum parameter values above are achieved at the weakest illumination, to investigate its stability under this condition, we thereupon measured the timeresolved photoresponse (Iph−t) to a pulsed weakest illumination of P = 25 μW/cm2 at Vds = 5 V, and the corresponding results are shown in Figure 3a. For every separate Iph−t curve, our devices demonstrate definite on/off states and maintain reproducibility of their photoresponse in a series of pulsed illumination stimulations. The response time also represents a critical parameter for high-performance photodetectors, with a fast detection speed that can greatly expand their practical application. In Figure 3b, we recorded the dynamic response of our device in a complete on/off cycle with a high temporal resolution. Interestingly,

separation of photogenerated electron−hole pairs but also reduce the electron−hole recombination rate and thus benefit to photocurrent and also response speed.40,43 Moreover, extracted from Figure 2c, the current on/off ratio at various voltage is >500 (27 mW/cm2), which indicated that the asprepared device shows good light-switching behavior. Besides, the corresponding I−V characteristics of the pristine In2Se3 phototransistor are presented in Figure S4a,b. The source-drain current (Ids) of these two devices increases significantly under illumination. To explore the quantitative dependence of the photocurrent on the illumination intensity, the photocurrent (Iph = Ilight − Idark) was measured at Vds = 5 V as a function of the laser power density (P). As presented in Figure 2d (blue), we observed that the photocurrent improves obviously after coating NDs. Furthermore, the photocurrent increases following a power law equation of Iph ∼ Pα, where P is the light power density and α represents the index of the power law. Fitting the measured photocurrents, the value of α1 = 0.56 for the pristine In2Se3 and α2 = 0.72 for the hybrid NDs−In2Se3 are achieved. Here, the deviation from the ideal index of α = 1 implies the route of the loss of the photoexcited carrier by a recombination.44,45 The fact that α1 < α2 indicates that NDs could suppress the recombination probability of the photoexcited carriers, leading to the increase of the photocurrent. The photodetector performance critically depends on the parameters, such as photoresponsivity (Rλ), external quantum efficiency (EQE), detectivity (D*), response speed, and stability. According to Rλ = Iph/PS, defined as the ratio of the generated photocurrent (Iph) in response to incident light power density (P) impinging on the detector (S, effective area of photosensitive region),46 we obtained the 3D responsivity map of these two photodetectors, described in Figure 2e and Figure S4c. Clearly, the photoresponsivity increases with the increase of bias voltage (Vds) and decreases with the increase of incident light intensity. Remarkably, the pristine In2Se3 device achieves a decent photoresponsivity of 17.4 A/W under 532 nm light illumination with a power density of 25 μW/cm2 at Vds = 5 V. This value can be enhanced to 33.6 A/W after hybridization the NDs, as shown in Figure 2d (magenta). These values are better than other layered material based photodetectors.19,47,48 It is notable that the Rλ displays a dramatic increase with the decrease of the light intensity, which may due to the reduction in the density of electron trapping states under high illumination intensity which led to the saturation of the photoresponse15 while, even under the weakest light irradiation (25 μW/cm−2), these two devices E


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Figure 4. (a) The on/off switching response as a function of the bias voltage under illumination of 532 nm and 27 mW/cm2. The inset shows the photocurrent and on/off ratio (Ilight/Idark) as a function of bias voltage. (b) Operational stability of our NDs−In2Se3 photodetector. Illumination of P = 27 mW/cm2 and Vds = 0.2 V are used here.

there are no sampling points at the rise and decay edges of pristine In2Se3 device, indicating that the rise/fall time of this device is faster than the measuring limit of the present setup (8.3 ms). While for the NDs−In2Se3 device, after the illumination was turned on or off, the photocurrent rapidly rise/fall more than 75% in the initial 8.3 ms, followed by a relatively slow tail. Defining the rise/fall time as the current increased/decreased from 0/100% to 80/20% of the stable current, the rise/fall time can be calculated to be less than 16.6/ 24.9 ms. The relatively slower rise/fall time compared with pristine In2Se3 based photodetector was attributed to the slow recombination process of the photoexcited electron−hole pairs in NDs−In2Se3. The detailed reason will be discussed below. Notably, the response speed of the NDs−In2Se3 device is superior to many other nanostructure based photodetectors.52−54 For a clear comparison, Table S1 summarizes the relevant performance parameters of other recently developed photodetectors based on layered materials. The measured detectivity of our NDs−In2Se3 device is much larger than most of the previously reported values for layered material based photodetectors. Note that although many other photodetectors show higher responsivity or larger current on/off ratio than ours,9,10,55 they always suffer from either long response time or low responsivity, which is difficult to balance high responsivity, fast response speed, and superior current on/off ratio simultaneously. Our device integrated competitive detectivity, fast response speed, and superior current on/off ratio simultaneously, making it possess great promising applications in commercial optoelectronic systems. Herein, we can conclude that NDs can be introduced to improve the photodetection performance of the pristine In2Se3

photodetector. It is worth noting that pure NDs are verified to show no photoresponse performance; relevant explanations will be discussed below. In the following step, we further test the photoresponse characteristics of the hybrid NDs−In2Se3 device. The voltage-dependent photocurrent has been investigated in Figure 4a by varying the bias voltage from 0.2 to 5 V. The output photocurrents present definite on/off switch, impressive consistency, and long-term repeatability under a wide range of bias voltages. In particular, the corresponding photocurrents were extracted and are plotted as inset in Figure 3a as blue spheres; these values exhibit positive related to the bias voltage, which on account of an increasing Vds can enhance the separation efficiency of photoinduced electron−hole pairs and shorten their transit time to reach the electrodes by providing a stronger electric field, thus reducing the possibility of recombination. This feature grants smooth modulation of the photoresponse for multifunctional applications. Meanwhile, the voltage-dependent current on/off ratio demonstrated in Figure 3b (magenta spheres) indicates that we can extracted a relatively high current on/off ratio of 545 at Vds = 5 V, and this value can reach ∼2285 under low operating voltage. The high on/off ratio could have great potential in high-sensitivity applications. In addition, the photostability is also an important index to evaluate the practical applicability of a photodetector. As depicted in Figure 4b, no severe deviation was observed after 50 cycles of operation in the duration time of ∼1200 s, pointing to the excellent reliability and stability of our NDs−In2Se3 photodetector. Broadband Spectral Response of the Constructed Photodetectors. Previously, all measurements were made at a wavelength of 532 nm. To ascertain further the feasibility of F


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Figure 5. Broadband photodetection from UV to NIR for the NDs−In2Se3 photodetector. Switching behavior at Vds = 0.2 V under various illuminations with wavelengths of (a) 370 nm, 23 mW/cm2, (b) 447 nm, 40 mW/cm2, (c) 635 nm, 45 mW/cm2, (d) 808 nm, 34 mW/cm2, (e) 1064 nm, 47 mW/cm2, and (f) 1550 nm, 43 mW/cm2.

Figure 6. (a) UV−vis−NIR absorption spectra of In2Se3 and NDs−In2Se3. (b) Tauc plots of In2Se3 and NDs−In2Se3. (c) C 1s XPS spectra of NDs− In2Se3. (d) FTIR spectrum of the synthesized NDs.

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Figure 7. (a) PL spectra of NDs and NDs−In2Se3. (b) UPS spectra near the onset part of In2Se3. (c) VB XPS spectra of In2Se3. (d) Band energy diagrams of NDs and In2Se3.

where μ is the carrier mobility, W is the channel width between the parallel electrodes, E is the applied electric field between electrodes, n is the excess carrier density, and ηabs is the material’s incident light absorption depth.55 Therefore, the photocurrent is closely concerned with the excess carrier density (n) and the abilities of light harvesting (ηabs). At first, we compare the diffuse reflectance spectra of pristine In2Se3 and hybrid NDs−In2Se3 active layer in Figure 6a. The prepared NDs are observed to exhibit broad spectrum absorption ability (even in NIR region). Moreover, the NDs−In2Se3 possesses a stronger spectrum absorption ability than that of pristine In2Se3, endowing it an improved lightharvesting depth (ηabs). According to the Tauc plots shown in Figure 6b, the bandgaps of In2Se3 and NDs−In2Se3 can be calculated to be ∼1.20 and ∼1.14 eV, respectively. The narrowing of the bandgap of NDs−In2Se3 reveals the synergistic effect between NDs and In2Se3 in the hybrid NDs−In2Se3. Then, how does this synergistic effect work? In the next step, we characterize the prepared NDs in detail. To verify the chemical states of C elements in the prepared NDs, quantitative XPS analysis for C 1s was performed as shown in Figure 6c. The fitting of the binding energy peak reveals that it could be divided into four Gaussian peaks, which were sp2 CC (284.4 eV), sp3 C−C (285.4 eV), C−O (286.9 eV), and CO (288.7 eV).57 The sp3 C−C subpeak indicates the apparent transformation of non-diamond to diamond phase, while the sp2 CC subpeak illustrates that there still presents a small sp2 C fraction on the NDs.25 And the C−O and CO subpeaks reveal that abundant functional groups are detected in NDs, especially C−O and CO. In order to further discern the functional groups on NDs, Fourier

broadband photodetection, we measure the spectral response of our devices across wavelengths of 370−1550 nm. In Figure S6, we depicted the measured switching behavior of pristine In2Se3 device under various illuminations. One can observe an obvious photocurrent and a pronounced reproducibility for wavelengths across from 370 to 1064 nm, confirming its broadband response capability. For wavelength of 1550 nm, the photocurrent is still discernible but very weak. It may been suggested that the intrinsic defects and native oxides that grow at the surface of the In2Se3 film in ambient conditions could act as efficient energy converters of incident light that supports this response.56 For the NDs−In2Se3 device, as shown in Figure 5, we can observe a clear photoresponse for all illumination cases, even for wavelength of 1550 nm. This property means that the synergistic effect between NDs and In2Se3 film may broaden the response range of In2Se3 detector. NDs-Mediated Photodetection Mechanism. Then, we come to investigate the mechanism on the photodetection performance enhancement of the hybrid NDs−In2Se3 photodetector. Under illumination, the In2Se3 film absorbs photons with energy larger than that of the In2Se3 bandgap (Eph > Eg) and generates a large number of electron−hole pairs according to the photoconductive effect.43 Then these photogenerated electron−hole pairs would be separated and extracted by Au electrodes in opposite directions under the applied electric field. In this process, photocurrent was generated, and the value followed the equation Iph = qμWEnηabs

(1) H


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Figure 8. (a, b) Synchronous measurements for AFM topography and CPD images in the dark. (c) Plot of the CPD measured along the blue dashed line depicted in panel b. (d, e) Synchronous measurements for AFM topography and CPD images under illumination. (f) Plot of the CPD measured along the blue dashed line depicted in panel e.

NHE). Thus, the conduction band (CB) position was considered to be −0.56 eV (vs NHE) by deducting the value of bandgap (∼1.2 eV) from the VB maximum. Given that the VB and CB position of NDs was determined to be 1.45 and −0.76 eV (vs NHE), respectively,25 we sketch simple bandgap models of NDs and In2Se3 in Figure 7d. The band model of NDs with continuous bandtail and midgap states is attributed to the existence of functional groups on the surface. In this bandgap model, photoexcited electron−hole pairs in NDs can transfer to the In2Se3 film due to the lower energy levels for electron and hole in In2Se3. Since the transfer rates of the electrons and holes are different, the numbers of electrons and holes to In2Se3 are different. To further verify the light-induced charges transfer in the NDs−In2Se3, we utilize Kelvin probe force microscopy (KPFM) to quantitatively analyze the changes in surface potential under illumination.59 The contact potential difference (CPD) between the AFM tip and the local area of In2Se3 or NDs can be given by

transform infrared spectroscopy (FTIR) was introduced as shown Figure 6d. Consistent with the XPS result, the dominating functional groups are still C−O groups, while O−H, C−H, and COOH bonds are also found. These functional groups on the surface of NDs are suggested to act as photoluminescence (PL) centers.58 Therefore, PL measurements of NDs and NDs−In2Se3 are carried out in the following step. As displayed in Figure 7a, by excitation with a 325 nm laser, two prominent peaks centered at ∼585 nm for NDs and ∼600 nm for NDs−In2Se3 are observed, which can be attributed to the band−band PL in NDs.25 The small red-shift of PL peak center in NDs−In2Se3 may further unveil the synergistic effect between NDs and In2Se3 film. Moreover, broad band PL emission lying in the NIR region is also observed in the NDs. It means that the functional groups on NDs can expand the PL emission wavelength, which may be responsible for its NIR absorption property. The detailed PL mechanism can be found in our previous work.58 Besides, the PL intensity in NDs−In2Se3 is much weaker than that of pristine NDs, indicating that the recombination of the photoexcited electron−hole pairs in NDs has been strongly suppressed. With this, there may be a charge transfer process between NDs and In2Se3 film. In this case, the band structures of NDs and In2Se3 play an important role for the carrier transfer. UPS measurements were performed to further study the energy level of our In2Se3 film. The UPS spectra at low kinetic energy region are shown in Figure 7b. By linearly extrapolating the low kinetic energy onset part, the work function of our In2Se3 can be estimated to be 4.39 eV (−0.11 eV vs NHE). Thereupon, as presented in Figure 7c, by linear extrapolation of the leading edges of VB XPS spectra to the base lines, we can acquire the valence band (VB) position of In2Se3 of 0.64 eV (vs

CPDIn2Se3 = Φtip − ΦIn2Se3

(2)

CPDNDs = Φtip − ΦNDs

(3)

where Φtip, ΦIn2Se3, and ΦNDs are the work functions of the tip, In2Se3, and NDs, respectively. The synchronous measurements for AFM topography and its compounding CPD distribution of the hybrid NDs−In2Se3 in the dark and under white light illumination are shown in Figure 8. The Fermi levels difference (ΔEf) between the In2Se3 and NDs is obtained by measuring the ΔCPDNDs−In2Se3 on average, which is defined as I


Research Article


unseparated electron−hole pairs quickly recombine, leading to a quick decay of the photocurrent. In contrast, it takes more time for the spatially separated electron−hole pairs to meet their counterparts and recombine, which leads to the slower decay rate of the photocurrent. As a consequence of the improved light-harvesting depth, broader spectrum absorption ability, and increased excess carrier density of NDs−In2Se3, according to the eq 1, a superior performance NDs−In2Se3 photodetector can thus be achieved. Note that the isolated NDs (not attached on In2Se3) exhibit no photodetection performance. Since photoexcited electrons and holds will localize at the surface of the NDs, they may recombine quickly as the surface functional groups would act as the PL centers. Therefore, the synergistic effect of hybrid multilayer In2Se3 with nanodiamonds is crucial for the highly sensitive photodetector.

ΔEf = ΦIn2Se3 − ΦNDs = CPDNDs − CPDIn2Se3 = ΔCPDNDs − In2Se3

(4)

As shown in Figure 8b, owing to the work function of NDs being smaller than that of In2Se3, the positions of the NDs are discernible in the CPD image in the dark. From a typical line profile of the CPD (Figure 8c), an apparent ΔEf ∼ 9.3 mV can be produced. Under illumination, the positions of the NDs become blurred (Figure 8e), while the value of ΔEf decreases to about 5.2 mV (Figure 8f). This significant change of ΔEf between dark and illumination conditions can be ascribed to the obvious charges transfer occurring in the NDs−In2Se3 under illumination. Under illumination, the photogenerated electrons in NDs can migrate more promptly to the In2Se3 film than the photogenerated holes; the quasi-Fermi levels of these two materials would shift in opposite directions, as shown in Figure S7. In other words, the Fermi levels difference between In2Se3 and NDs narrows under illumination, corresponding to decrease in ΔEf. While, under illumination, if photoexcited holes in NDs flow into the In2Se3 film more quickly than that of electrons, or that electrons in conduction band of In2Se3 transfer to the trap states in NDs, the Fermi levels difference between In2Se3 and NDs would widen, which is in contrast with the experimental data. Based on the above analyses, the NDs-mediated photodetection mechanism can be schematically described in Figure 9. Under illumination with wavelength ranging from UV (370

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CONCLUSION In summary, we have introduced NDs to improve the photodetection performance of multilayer In2Se3 for the first time. The prepared NDs have been demonstrated to be mixed with a small sp2 C fraction and covered by abundant functional groups, resulting in its broad spectrum absorption property. Impressively, the photodetector based on NDs−In 2 Se 3 exhibited outstanding photodetection capabilities, including a remarkable detectivity of 5.12 × 1012 jones, plus an ultrafast response/recovery time less than 16.6/24.9 ms for the rise/fall along with a relatively high current on/off ratio of ∼2285. These parameters balance high detectivity, fast response speed, and decent current on/off ratio simultaneously. They are superior to the pristine In2Se3 device and better than most reported layered material based photodetectors. In particular, these remarkable performances exhibited an excellent long-term stability. Further, a possible NDs-mediated photodetection mechanism has been proposed. Probably, these excellent performances are arising from the synergistic effect between NDs and the In2Se3 film. They can greatly enhance the broad spectrum absorption and promote the injection of photoexcited carrier in NDs to In2Se3. These results suggested that NDs are promising for ameliorating the photodetection property of layered materials.

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Figure 9. Electron−hole separation mechanism of the hybrid NDs− In2Se3 during light illumination.

ASSOCIATED CONTENT

S Supporting Information *

nm) to NIR (1550 nm), electron and hole pairs are excited in In2Se3 and NDs. The pairs in In2Se3 may be separated rapidly under the applied electric field, leading to its decent photodetection property. For the NDs, the small sp2 C fraction on the NDs can drag the photoexcited carriers to the surface25 and then transfer to the In2Se3 film due to the lower energy levels for electron and hole in In2Se3. The photogenerated electrons in NDs can migrate more promptly to the In2Se3 film than the photogenerated holes, leading to fast increase the excess carrier density (n) in the In2Se3 film. We called it the synergistic effect between NDs and the In2Se3, which is the main cause for the high-performance photodetection. In addition, the slower injecting holes results in the slow increase of the photocurrent, which indicates the slower rise time. It is noted that the inconsistent migration rate of electrons and holes leads to excess electrons in In2Se3 and excess holes in NDs. That is, a portion of photogenerated electron−hole pairs are spatially separated. When the light is turned off, the spatially

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06531. Summary of reported photodetectors, size distribution histogram of NDs, XRD patterns of the graphite powders and NDs, 3D AFM images of In2Se3 and NDs−In2Se3 film, I−V curve and corresponding 3D responsivity map of the pristine In2Se3 photodetector, current on/off ratio of NDs−In2Se3 photodetector, broadband photodetection of In2Se3 photodetector, and the energy band diagrams for NDs and In2Se3 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (G.Y.). Notes

The authors declare no competing financial interest. J


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ACKNOWLEDGMENTS The National Natural Science Foundation of China (91233203) and State Key Laboratory of Optoelectronic Materials and Technologies supported this work.

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Synergistic Effect of Hybrid Multilayer In2Se3 and Nanodiamonds for

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