Improvements in the Performance of a VisibleâNIR Photodetector

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Article Cite This: ACS Omega 2019, 4, 6180−6191

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Improvements in the Performance of a Visible−NIR Photodetector Using Horizontally Aligned TiS3 Nanoribbons Mohammad Talib, Rana Tabassum, Abid, Saikh Safiul Islam, and Prabhash Mishra* Centre for Nanoscience and Nanotechnology, Jamia Millia Islamia (A Central University), New Delhi 110025, India

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S Supporting Information *

ABSTRACT: We report the fabrication and characterization of visible and near-infrared-resistive photodetector using horizontally aligned titanium tri sulfide (TiS3) nanoribbons. The fabrication process employed micro-electromechanical system, photolithography and dielectrophoretic (DEP) methods. The interdigitated electrodes (IDE) fingers were fabricated using photolithography and thin-film metallization techniques onto the Si/SiO2 substrate, and then TiS3 nanoribbons were horizontally aligned in between IDE using DEP. The fabricated device was first characterized in absence of light and then, the photodetector-based characteristics were obtained by illuminating it with fiber-coupled laser beam. These characteristics were optimized by varying wavelength and power density of the laser beam. The present photodetector shows a maximum responsivity of 5.22 × 102 A/W, quantum efficiency of 6.08 × 102, and detectivity of 1.69 × 109 Jones. The switching times, i.e., response and recovery times were found to be 1.53 and 0.74 s, respectively, with 1064 nm wavelength and 3.4 mW/mm2 power density of the laser beam. Also, the effect of O2 adsorption on nanoribbons has been studied and it is found that adsorbed O2 acts as electron acceptor and decreases the conductivity of the photodetector. Experimentally, it is found that the photoresponse of the horizontally aligned TiS3 nanoribbons is better than that of a randomly oriented TiS3 nanoribbon-based photodetector. Finally, the performance of the present photodetector was compared to that of the previous ones that were found to outperform the reported ones. The additional advantages of the photodetector include excellent stability and portability from which it may be concluded that TiS3 nanoribbons can be a promising candidate for application in nanoscale electronic and optoelectronic devices. performance in comparison with the pristine film.12 Recently, photodetectors based on two-dimensional (2D) materials with vertical structures, like V-HfS2,17 V-MoS2/Si,16 V-SnS2,15 and V-MoS2,19 have been reported, which exhibit excellent photoresponse and high photosensitivity. Also, many UV detectors with enhanced performance have been reported, like UV−visible photodetectors based on integrated Se/n-Si p−n heterojunctions,11 large-area ZnS nanobelt-based UV-light sensor with enhanced photocurrent and stability,13 selfpowered UV photodetector based on a crossed ZnO nanofiber array homojunction.20 All of these factors cumulate in a way so as to enhance the performance and activity of a photodetector. At the same time, a 1D material-based device overcomes some limitations, such as the effect of dark current, interference of the surrounding parameters, and the requirement for complicated and sophisticated purification/fabrication procedures. In view of the aforementioned discussion, onedimensional (1D) materials hold great promise in the avenue of the photodetector and thus are an active and useful area of research.

1. INTRODUCTION Over the last decade, nanostructured materials have attracted attention from the research and industrial community due to their extremely interesting properties and applications in areas such as sensing technology, medical diagnostics, catalysis, drug delivery systems, bioprocessing, optoelectronic devices, etc. One of the prominent optoelectronic devices may be photodetectors. Various nanostructured materials have emerged as efficient visible as well as near-infrared (NIR) photodetectors with improved sensitivity, photoresponse, stability, etc. The reason being the nanostructured materials provide quantum, optical confinement effects and large surface areas, which lead to a higher extent of light absorption, which in turn gives a better detection capability.1−5 Published results support the development of a photodetector using onedimensional (1D) nanostructured materials as a wonderful tool to enhance the performance of optoelectronic devices.6−10 Moreover, size and shape of the nanostructured materials can be tailored in terms of nanosheets, nanotubes, nanobelts, nanoribbons, etc. concerning a better detection capability.11−20 This assembly of nanostructured materials is found to be quite advantageous in enhancing the performance of a device in several prospects. The highly ordered nanostructure-based photodetectors displayed significant enhancement in device © 2019 American Chemical Society

Received: November 3, 2018 Accepted: January 17, 2019 Published: April 2, 2019 6180

DOI: 10.1021/acsomega.8b03067 ACS Omega 2019, 4, 6180−6191

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and methods also suffer from several limitations. These limitations include complicated fabrication procedures, usage of large quantities of chemicals and reagents, prolonged duration of the analysis, requirement of skilled personnel and equipments, and moreover, the involvement of an ultrahighsensitive lab. Thus, the objective of a simple, fast, accurate, sensitive, and cost-effective methodology for fabrication and characterization of photodetectors that work both in visible and NIR regions of the electromagnetic spectrum remains unattained. The present study illustrates an efficient strategy to achieve these aims. In this work, we describe a horizontally aligned TiS3 nanoribbon-based visible and NIR photodetector on the Si/ SiO2 substrate. First of all, TiS3 nanoribbons was prepared using chemical vapor transport (CVT) method. Before fabricating the detector, the structural and morphological properties of the TiS3 nanoribbons were investigated using Xray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energydispersive X-ray spectroscopy (EDS) characterization techniques. Then, for the fabrication of the detector, TiS 3 nanoribbons were horizontally aligned in between titanium/ gold interdigitated electrodes (IDE) on Si/SiO2 substrate using dielectrophoresis (DEP) method. Finally, in the fabricated device, photodetector-based characteristics were obtained by collimating it with a fiber-coupled laser beam of varying wavelength and power density. The output photoresponse, responsivity, (Rλ), detectivity (D), and quantum efficiency (QE) of the photodetector were determined using the experimental characteristics of the photodetector. The response time and recovery time of the fabricated photodetector were also evaluated. Finally, the performance of the present photodetector was compared to that of the reported ones.

Among various layered materials, black phosphorous, graphene, dichalcogenides (MoS2, WS2, etc.) and trichalcogenides (TiS3, TaS3, NbS3, NbSe3, etc.) possess weak out-ofplane bonding between layers, which allows the exfoliation of their bulk down to a single layer using simple exfoliation techniques.21−29 Among these, TiS3 have established an extremely important place in future electronics and optoelectronic devices. TiS3 possesses modest band gap; higher carrier mobility; and better anisotropic, electronic, and optical properties, as well as it is found to be a chemically stable material; therefore, TiS3 can be a suitable candidate for optoelectronic devices.30−39 At the same time, nanostructures of TiS3 can be easily integrated with conventional silicon/ germanium materials as well as with the optical technologies for their fabrication protocols. On comparing quasi 1D nanostructures of TiS3 with its higher dimensions, it is observed that this nanostructure material offers a combination of high electric-field tunability and large surface area, which is very beneficial in fabricating complex devices such as detectors and sensors. Apart from TiS3, the most common member of the layered family is graphene, which has been considered as most promising candidate for the development of optical devices and is also used in semiconductor industries, the reason being their higher carrier mobility. But at the same time, the applications of graphene in electronic and optoelectronic devices are limited due to the absence of an electronic band gap. This limitation becomes very prominent where controllable operations are required.40−48 Contrary to this, the properties of TiS3 are such that it can be efficiently be used in the fabrication of optical devices.49−51 TiS3 possesses a robust direct band gap of approximately 1 eV, high optical responsivity, and also relatively higher in-plane electron mobility. In addition to this, the TiS3 monolayer possesses strong anisotropies in electrical and optical properties, which is very useful in fabricating next-generation electronic devices, like high-mobility transistors. The general methods for fabrication of trichalcogenides are chemical vapor transport (CVT) and sulfurization of their bulk material. The product obtained using these methods is in the form of layers formed of quasi 1D whiskers and nanosheets or nanoribbons.52,53 At low growth temperatures such as 400 °C, TiS3 shows a morphology of flowerlike nanostructures and nanosheets. At higher growth temperatures, TiS3 grows in the form of nanoribbons. A report has been found on the isolation and exfoliation of the bulk TiS3 material obtained at varying growth temperatures. They reported that TiS3 nanosheets that have been grown at slightly lower temperature, show higher electron mobility (≈73 cm2/V s) whereas the TiS3 nanoribbons have lower mobilities but show a superior electric field and optical tenability.9,54−57 This is due to the differences in the electronic properties of the TiS3 samples grown at lower temperature and at higher temperatures, which is obtained by density functional theory calculations. This work constitutes a significant step in exploring trichalcogenide (TiS3) in electronic applications such as detectors wherein large surface area is required. There are various materials and methods already reported to fabricate and characterize efficient and inexpensive photodetectors. Some of the reported methods include the use of multilayer WS2,58 multilayer InSe,59 graphene,60 multilayer GaSe,61 microstructured Si,62 In2Se3 nanosheet,63 monolayer MoS2,6464 GaTe Flakes,65 ZnO nanowires,66 etc. for efficient and sensitive photodetectors. Although the reported materials and methods provide a good photoresponse, these materials

2. EXPERIMENTAL SECTION 2.1. Preparation of TiS3 Nanoribbons. TiS3 nanoribbons were synthesized by a solid-gas reaction between titanium powder (2 g, good fellow, 99.5% purity) and sulfur gas produced by heating of sulfur powder (6 g, Merck, 99.9% purity). This reaction was carried out in a vacuum-sealed ampoule, and vacuum is used to remove impurities, such as atmospheric oxygen and water, and also prevent oxidation of titanium during heating. Prior to this reaction, first Ti disks were etched in an acidic mixture of hydrofluoric acid and HNO3 (4:30 wt %), which cleans the surface of Ti disks and removes any oxide impurities. Ampoules were then heated at a rate of 250 °C per h and kept at 500 °C for 20 h (temperature and time of sulfuration have been previously optimized) in a horizontal furnace. After approximately 20 h of growth process, the ampoule is cooled in ambient condition; TiS3 nanoribbons were then obtained from the ampoule. The structural, morphological, compositional, as well as optical properties of the fabricated samples were examined by X-ray diffraction (XRD), Field-emission scanning electron microscopy ( FESEM), Energy-dispersive X-ray spectroscopy (EDS), High resolution transmission electron microscopy (HRTEM), and UV−Vis spectroscopy techniques. 2.2. Fabrication of a Horizontally Aligned TiS3 Nanoribbon-Based Detector. For the fabrication of a TiS3 nanoribbon-based detector, an appropriate amount of TiS3 nanoribbons were suspended in ethanol, followed by sonication for 30 min. The oxidized Si wafer substrate having 6181


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medium, i.e., |εp*| > |εm*|, the force will push the particle into the region of higher electric-field intensity and vice versa. In the present suspension of TiS3 nanoribbons, as a result of DEP force, TiS3 nanoribbons get pushed into the region of higher electric field. The TiS3 nanoribbons get deposited bridging the electrodes because near to the electrodes, the electric-field intensity was found to be the highest. This method is found to be very beneficial in the fabrication of fully packed TiS3 channels using a suspension of an even low concentration of TiS3 nanoribbons. 2.3. Implementation of a TiS3 Nanoribbon-Based Photodetector. In the fabricated TiS3 nanoribbon-based device, for photodetector-based characterization, the device was illuminated with a fiber-coupled laser light of varying wavelengths and power density. The spectral responses of the photodetector characteristics were recorded at different wavelengths and power of the laser beam. The current− voltage (I−V) characteristics of the fabricated photodetector were measured by Keithley SCS4200 (Keithley Instruments Inc.).

a SiO2 top layer (300 nm) is used as a substrate. Then, to fabricate the metallic electrodes, Ti/Au (8:100 nm) was deposited on both ends of the Si/SiO2 substrate using standard photolithography and liftoff process. Again, in between Ti/Au interdigitated electrodes, the suspension of TiS3 nanoribbons was dropped and by application of alternating current (AC) voltage, the suspended TiS3 nanoribbons got aligned in between interdigitated electrodes. This horizontal alignment of TiS3 nanoribbons in between (Ti/Au) interdigitated electrodes can be understood by the well-known “dielectrophoresis” (DEP) technique.67 In the typical DEP technique, a nonuniform electric field is applied to the suspension of TiS3 nanoribbons, which results in the polarization of suspended particles within the corresponding solvent. There will be some gradients in the electric field that force the alignment of the TiS3 particles (dipoles) with the local field lines and push these particles toward regions with the most intense fields. In the initial studies, DEP has been extensively used to assemble carbon nanotubes68 or graphene69−71 between metallic electrodes; later, its use to align the 2D and 1D nanostructures has been explored.72 In our implementation of the DEP technique to align the TiS3 nanoribbons, we have assembled an experimental set up as shown in Figure 1. In Figure 1, Ti/Au interdigitated

3. RESULTS AND DISCUSSION The X-ray diffraction (XRD) pattern of the TiS3 nanoribbons obtained using smartlab Rikagu with Cu Kα radiation (λ = 1.54 Å) in θ−2θ configurations is shown in Figure 2a. Deposits formed using monoclinic TiS3 as a unique crystalline phase can be well identified using the XRD diffraction pattern in Figure 2a. The XRD pattern is found to be consistent with JC-PDSICDD 15-0783.49 All diffraction peaks are found to be narrow, which confirms that the material is well crystallized. Lattice parameters (a, b, c, α, β, γ, and V) determined from XRD patterns are listed in Table 1, which are found to be comparable with the reported ones.49 Therefore, it is confirmed that pure TiS3 nanoribbons have been formed using our fabrication method discussed in the Experimental Section. From the optical transmission spectra of the fabricated TiS3 nanoribbons, its band gap can be measured using Tauc’s relationship. According to theoretical and practical results, TiS3 exhibits direct interband transitions and with photon energy (hν), it is found to obey the given relation

Figure 1. Sketch of the DEP technique to horizontally align TiS3 nanoribbons in between the Ti/Au interdigitated electrode onto the Si/SiO2 substrate.

electrodes (IDE) were grown on a Si/SiO2 substrate and an AC voltage has been applied between Ti/Au IDE. The AC voltage is a 1 MHz sinusoidal signal of 10 V peak-to-peak, and the electrodes are separated by a 5 μm distance. A drop of the colloidal suspension of TiS3 nanoribbons is dropped in between the electrodes as soon as the electric field is generated by the oscillating voltage. This electric field polarizes the TiS3 nanoribbons with respect to the solvent. The gradients in the electric fields in between the TiS3 nanoribbons and the corresponding solvents push the TiS3 nanoribbons toward the region of the highest electric-field intensity. This can be better understood using a DEP force acting on tubelike particles and is given by the following equations73 π FDEP = r 2lεm Re(fCM )∇|E|2 (1) 2 É ÅÄÅ * Ñ ÅÅ εp − εm* ÑÑÑ ÑÑ fCM = ÅÅÅ ÅÅ ε * ÑÑÑ m (2) ÅÇÅ ÑÖÑ

αhν = A(hν − Eg )1/2

(3)

where hν is photon energy, α is the absorption coefficient, A is a constant, and Eg is the band gap. From eq 3, it is worth mentioning that by plotting (αhν)2 against photon energy hν, the band gap value of TiS3 can be determined by extrapolating the straight line portion at α = 0, as shown in Figure 2b. In this figure, the variation of (αhν)2 values as a function of energy of the incident radiation has been depicted. The band gap energy (Eg) of TiS3 was obtained by extrapolation method. It may be noticeable from the figure that on extrapolating the straight line portion of (αhν)2, the curve meets the x axis (energy) at 1.1 eV, which is considered as the optical band gap of TiS3. Once the formation of TiS3 was confirmed, the morphology of the TiS3 nanoribbons was also examined from a scanning electron microscopy (SEM) image. The suspension of the prepared TiS3 sample is shown in Figure 2c. In the SEM image shown in Figure 2d, 1D ribbonlike structures are clearly revealed. Then, the dimension, structure, and orientation of the TiS3 nanoribbons has been confirmed using the trans-

where εm* and εp* are the complex permittivities of the suspension solvent and the suspended particle, respectively, E is the applied electric field, and f CM is the Clausius−Mossotti factor that depends on the complex permittivities of the suspension solvent and the suspended particle. If the suspended particle has a polarizability higher than that of the 6182


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Figure 2. (a) X-ray diffraction (XRD) pattern, (b) Tauc’s relationship showing the band gap, (c) image of the suspension sample, (d) SEM image, (e) TEM image, (f) energy-dispersive X-ray spectroscopy, (g) HRTEM image, and (h) selected area electron diffraction (SAED) pattern of the TiS3 nanoribbons.

of micrometers with smooth surfaces. The high-resolution TEM (HRTEM) image shown in Figure 2e and 2f depicts the uniform nanoribbon-like structures and clear lattice fringes of

mission electron microscopy (TEM) images shown in Figure 2e. TEM images confirm that the formation of TiS 3 nanoribbons has been obtained with lengths of several tens 6183


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Table 1. Lattice Parameters of TiS3 Obtained from XRD Patterns and the Values Previously Reported in ref 49 parameter

JC-PDS

this work

a (Å) b (Å) c (Å) α (Å) β (Å) γ (Å) V (Å)

4.973 3.433 8.714 90 97.74 90 147.41

5.020 3.465 8.797 90 97.74 90 151.69

TiS3. It shows that the fringe spacing of the TiS3 nanoribbons is found to be 0.43. The corresponding d value [101] of the monoclinic structure has been obtained using the selected electron diffraction (SAED) pattern given in Figure 2g. SAED patterns also reveal that the fabricated TiS3 nanoribbons were single crystal in nature. Further, the energy-dispersive spectroscopy (EDS) spectrum given in Figure 2h reconfirms the formation of TiS3 and separate peaks of Ti and S have been clearly noticed. EDS spectrum clearly depicts that the product composition is close to the stoichiometric composition of TiS3 (Ti/S = 25.81:70.19 ≈ 1:3). In this way, using all of these characterization techniques, the formation of pure TiS3 nanoribbons with well-known dimensions and composition has been confirmed. Further, we have characterized the fabricated horizontally aligned TiS3 nanoribbon-based detector using SEM. The SEM image of TiS3 nanoribbon-aligned device is shown in Figure 3.

Figure 3. SEM image of an aligned TiS3 nanoribbon-based photodetector.

From the SEM images, it can be seen that the TiS3 nanoribbons have been well accumulated in the region between the electrodes, where the two electrodes face each other in such a way that only a small amount of the material gets deposited on the sides of the two electrodes or on the SiO2 surface. The width of the TiS3 nanoribbons and gap between the two gold electrodes are around 30 nm and 5 μm, respectively. Since the fabricated device works on the principle and characteristics of a typical photodetector, experiments were performed to obtain the voltage−current (I−V) characteristics of the photodetector by illuminating the device with fibercoupled laser beams (monochromatic light) of varying wavelengths and power density. First of all, in Figure 4a, a schematic illustration of the horizontally aligned TiS3 nanoribbon-based photodetector is presented, and then the mechanism of a photodetector using the adsorption/ desorption behavior is shown in Figure 4b. The photodetection mechanism is based on the adsorption and desorption processes near the metal/semiconductor/metal (MSM) interfaces. The adsorption/desorption process can

Figure 4. (a) Schematic illustrations of the TiS3 nanoribbon-based photodetector, (b) band diagram of TiS3 under dark and illumination conditions, (c) I−V characteristics of TiS3 nanoribbon photodetector illuminated with different wavelengths in light and dark condition, (d) I−V characteristics of the photodetector for different power densities of the 1064 nm wavelength of the laser beam.

be modulated by the distribution of the local electric field, leading to the detection of the photocurrent. The schottky barrier height acts as a blocking zone at two interfaces, which can effectively tune the barrier height by illuminating with fiber-coupled laser beam. It is clearly shown in Figure 4b that once the photodetector receives sufficient laser light energy over the depletion region at the MSM interface, it decreases 6184


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Figure 5. Variation of the output current for (a) 635 nm, (b) 785 nm, and (c) 1064 nm wavelength of the laser beam as a function of time; (d) variation of the output current as a function of time at 1064 nm wavelength with different powers of the laser beam; (e) photocurrent as a function of power of the incident light for the data of figure (d), the curve can be fitted using power law; (f) variation of responsivity and quantum efficiency of the photodetector as a function of power density of the 1064 nm wavelength laser beam; (g) variation of detectivity as a function of power density of the 1064 nm wavelength laser beam; (h) typical response time and recovery time characteristics of TiS3 nanoribbon photodetector illuminated with 1064 nm wavelength of the laser beam.

the Schottky barrier height (or width) (δ). Subsequently, electrons jump into the conduction band from the valence band, creating holes in the valence band. These photo-

generated holes start to migrate to the surface and discharge the adsorbed oxygen ions (h+ + O2− → O2). Therefore, the electron is released from the oxygen ions and infused into the 6185


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were measured by periodically switching the three laser lights to ON and OFF at an applied voltage of 12 V. Clearly, the photodetector showed reversible switching between low- and high-conductance states with good stability and reproducibility. This shows that the photodetector is stable and repeatable in nature. It may be noticeable from Figure 5a−c that the time taken by the photodetector to switch from lowand high-conductance states is measured when the photodetector is irradiated by the laser beam of three different wavelengths, viz., 635, 785, and 1064 nm. Also, the trend of the curves of Figure 5 is such that within each cycle, the photocurrent increases fast during the transition from OFF to ON states and rapidly decreases to its baseline after the laser turns off, much faster than that for the response. After the fast spike, the photocurrent decays slightly before settling to a stable photocurrent. This slow decay in the photocurrent is resulted due to the accumulation of excess electrons on the nanoribbon surfaces. Rather than being trapped or extracted, these electrons undergo competitive recombination with the holes in the nanoribbons. Once the equilibrium of competitive separation and recombination of charge pairs is attained, the photocurrent becomes constant.75 To confirm the trend of Figure 5, we have measured more cycles of Figure 5a, given in Figure S3 of the Supporting Information. At the same time, since the “photocurrent” that is the more prominent parameter of the photodetector is found to be the highest for a 1064 nm wavelength. Therefore, we have also carried out the same experiments with varying powers of the 1064 nm wavelength. It may be noticeable from Figure 5d that the photocurrent is found to increase with increase in the power density of the laser beam but nearly the same time has been taken in switching from a low conductance to high conductance value for all power densities of the laser beam of 1064 nm wavelength. From Figure 5d, we can infer that the photocurrent of the horizontally aligned TiS3 nanoribbon-based photodetector shows strong nonlinear characteristics with the power density of the laser beam of wavelength 1064 nm. Therefore, the obtained photocurrent from Figure 5d has been plotted as a function of power density of the 1064 nm wavelength in Figure 5e. The experimental points have been plotted, and the curve is fitted exponentially; the obtained nature of the curve is found to be nonlinear. The obtained expression is of the form (I ∝ Pα) where “I” is the photocurrent, “α” is the absorption coefficient, and P is the power density of the laser beam of wavelength 1064 nm. Fitting of the curves resulted in α equal to 0.8. We note that such a nonlinear power density-dependent property of the photodetector, i.e., the nonunity exponent parameter (α) is a consequence of the complex processes, such as electron−hole generation, trapping, and recombination within the semiconductor.76 Using the photocurrent and dark current, we can measure prominent performance parameters of the photodetector: responsivity (Rλ), quantum efficiency (QE), and detectivity (D). By measuring the efficiency of electron transport and carrier collection, both responsivity (Rλ) and QE were measured. The responsivity measures the input−output gain of a detector system. Mathematically, it is defined as the ratio of difference of the photocurrent and the light power density per unit area of the TiS3 nanoribbons and can be determined from the following equation77

conduction band of the TiS3 nanoribbons, as shown in Figure 4b. Due to this, the conductance within the photodetector increases. This process continues until the photodetector receives sufficient light energy. But on further increasing the photons (laser light), the Schottky barrier width starts decreasing, resulting in increase of the discharge of the adsorbed oxygen ions and contributing a significant photocurrent. Hence, it may be concluded that the photocurrent passing through the Schottky contact is highly sensitive to the barrier height and width. 74 To further elaborate the mechanism, the I−V curve of the present TiS3 nanoribbonbased photodetector is plotted in Figure 4c, which was obtained when the fabricated device was exposed to the laser beam of different wavelengths from visible to near-infrared regions of the electromagnetic spectrum (635, 785, and 1064 nm) and also under dark conditions. It has been observed from Figure 4c that there was a little change in the obtained photocurrent when the wavelengths of the light source used were 635 and 735 nm. When the device was irradiated by a 1064 nm light, the photocurrent was found to be the highest, typically 12.76 μA, which is drastically large, i.e., nearly 73 and 23% higher than 635 nm (3.43 μA) and 735 nm (9.87 μA), respectively. The reason for the highest photocurrent at the 1064 nm wavelength has been explained using UV−vis absorbance spectra given in Figure S1 of the Supporting Information. In UV−vis absorbance spectra of TiS3, a peak is observed at approximately 1064 nm wavelength and due to an energy band gap match at this wavelength, the higher photoresponse of the detector was achieved. Also, the reason for decrement of photocurrent on decrement of wavelength (under same laser power density) is that the number of photons will decrease with the decrease of wavelength by the following formula14 N = Pλ /hc

(4)

where “P” is the power density of the laser beam, “λ” is the wavelength of the laser beam, “h” is Planck’s constant, and “c” is the speed of light in vacuum. Therefore, using eq 4, it is clear that if the wavelength of the laser beam is reduced, there would be lesser photons, resulting in lesser photogenerated charge carriers, hence in lower photocurrent. These photocurrents for all three wavelengths of laser light were obtained at the same applied voltage, typically 12 V. Since it has been depicted in Figure 4c that the maximum photoresponse has been obtained when the device is illuminated by a laser beam of 1064 nm wavelength, in Figure 4d, I−V characteristics were plotted for different power densities, typically 0.62, 2, 3.4, 4.95, and 6.9 mW/mm2 of the laser beam of 1064 nm wavelength and also for dark conditions. These measurements were taken for the same applied voltage of 12 V. From Figure 4d, it is noticeable that the obtained I−V characteristics are nonlinear in nature and the trend remains unchanged even with increased light intensity and the dark current was obtained as 2.51 μA, which is found to be extremely low. From this finding, it is well depicted that the electron carrier photogeneration efficiency is proportional to the absorbed photon flux. To further characterize the fabricated photodetector in terms of response time and recovery time, the experiments were performed to measure the photocurrent as a function of time. Figure 5a−c depicts variation of the photocurrent of the fabricated horizontally aligned TiS3 nanoribbon-based photodetector as a function of time for the three typical wavelengths 635, 785, and 1064 nm. The response time and recovery time 6186


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Table 2. Comparison of Responsivity of the Present Photodetector to that of Reported Ones material mixed-dimensionality TiS3/Si p−n junction multilayer WS2 multilayer InSe liquid-phase exfoliated TiS3 graphene multilayer GaSe microstructured Si In2Se3 nanosheet horizontally aligned TiS3 nanoribbons

Rλ =

ΔI PS

band gap (eV) 1.96 (direct) 1.4 (direct) 1.02 (direct) 2.11 (indirect) 1.1 (indirect) 1.3 (direct) 1.1 (direct)

operating wavelength (nm)

10 −3 10−5 10−4 10−3 10−2 10−2

34.8 9.2 3.47 3.8 1 1.7

1130 955 1064

1.19 × 102 3.95 × 102 5.22 × 102

electronic quantum efficiency

response time (s) 20 × 10 −3 5.3 × 10 −3 488 × 10−2 8±2 10−12

5.2

1.63 × 105 6.08 × 102

1.8 × 10−2 1.53

references 18 58 59 73 60 61 62 63 present work

the sheet density of electrons in the ground sub-band and the increase of the sheet density of electrons in the other subbands explain why the responsivity (or QE) decreases with increase of the illumination power density. Finally, the responsivity of the present TiS3 nanoribbon-based photodetector has been compared to that of the reported ones in Table 2 and found to outperform the reported ones. Therefore, it may be concluded that the present horizontally aligned TiS3 nanoribbon-based photodetector possesses relatively better photoresponse. Besides responsivity and quantum efficiency, “detectivity” is yet another parameter to characterize the fabricated photodetector. To evaluate the performance of the described photodetector, we can calculate the inferred detectivity (D), which is defined as the ability of the photodetector to detect weak optical signals. We assume here that the noise is limited by shot noise; detectivity “D” is calculated by the following formula18

(5)

Rλ eλ

× × × × × ×

>1050 635 885 405 800 590

where ΔI is the difference between the photoexcited and dark current, P is the light power density illuminated on the horizontally aligned nanoribbons, and S is the area of the nanoribbons illuminated by the laser beam. Putting these values in eq 5, responsivity can be measured at each power density. Similarly, quantum efficiency (QE) is another performance parameter to characterize the photodetector. The quantum efficiency is defined as the ratio of the number of carriers collected by the photodetector to the number of photons of a given energy incident on the photodetector. It is defined as the number of signal charge carriers created per incident photon. Mathematically, QE can be determined from the following equation78 QE = hc

responsivity (A/W)

(6)

D=

where λ is the exciting wavelength, h is Planck’s constant (h = 6.626 × 10−34 J s), c is the velocity of light (3 × 108 m/s), and e is the electronic charge (e = 1.6 × 10−19 C). Substituting Rλ, the obtained responsivities, at different power densities of the laser beam of 1064 nm wavelength in eq 6, QE can be measured at the same power density and wavelength of the laser beam. Figure 5f shows the variation of both the responsivity and QE of the photodetector with the power density of the laser beam in the range 0.62−6.9 mW/mm2. It may be observed from the figure that both responsivity and QE decrease with increase in the power density of the laser beam. In Figure 5f, the trend of both the curves is such that from 0.62 to 3.4 mW/ mm2, the rates of decrease of the responsivity and QE were very high but on further increasing the power density from 3.4 to 6.9 mW/mm2, the rate of decrement in the responsivity and QE start decreasing. The reason for this decrement in the responsivity (or QE) of the photodetector at the higher value of power density of the same wavelength of the laser beam can be explained by the density of electrons in the sub-bands. With increase in the power density of the incident laser beam, the density of electrons in the outer sub-bands starts to increase: first, increase in the density of electrons in the outermost subband becomes considerable, then next to the outermost subband, and so on until the ground sub-band. This occurs owing to the fact that the electrons are excited by photons from the ground sub-band to the outermost sub-band and then they are scattered by phonons to the other sub-bands. The decrease of

RλAd1/2 (2eId)1/2

(7)

where “Ad” is the effective area of the photodetector, “e” is the electron charge, and “Id” is the dark current. The obtained detectivity is found to be 1.69 × 109 Jones. Again, detectivity is measured for each of the power densities and has been plotted in Figure 5g. It may be noticeable from the figure that the nature of the detectivity curve is the same as that of responsivity and quantum efficiency, the reason of such a trend being the same as that explained in Figure 5f. Further, experiments have also been performed on the randomly oriented TiS3 nanoribbons for the same wavelength and power density of the laser beam. The variation of photocurrent as a function of time for randomly oriented TiS3 nanoribbons has been given in the Figure S2 of Supporting Information. From the measurements, it is clearly visible that the photocurrent for all three wavelengths is found to be lower in randomly oriented TiS3 nanoribbons in comparison to that of their horizontally aligned ones. The reason for improvement in the performance of a horizontally aligned TiS3 nanoribbonbased photodetector is attributed to efficient transport of photogenerated carriers in aligned ribbons over a random TiS3 nanoribbon network. In the case of aligned nanoribbons, there is avoidance of an inter-ribbon junction or bundling formation of ribbons. This bundling acts as a scattering center, which affects the electron-transfer properties of TiS3 due to which the number of photogenerated charge carriers at the electrode will be reduced, which will ultimately degrade the performance of 6187


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the photodetector. The other important advantage of the aligned nanoribbons is the lack of bending within the nanoribbons, whereas in the case of randomly oriented nanoribbons, there is a huge bending, which degrades the device performance. Furthermore, we have clearly mentioned the response and recovery time of the photodetector when it is irradiated with a 1064 nm wavelength of the laser beam at 3.4 mW/mm2 power density. The measurement has been shown by using one lowto-high switching cycle (first cycle) at a time. From Figure 5h, the dark current of the photodetector is found to be 2.51 μA, and the high photocurrent, 12.58 μA, which is measured under irradiation with the 1064 nm light at 3.4 mW/mm2 power density of the laser beam. The ratio of photocurrent to dark current is called as the ON/OFF ratio, which is found to be about 5.01. Further, the time taken for the current to increase from 10 to 90% of the peak value and vice versa is defined as the rise time or the response time and decay time or recovery time, respectively. Both the rise time and recovery time obtained for 1064 nm wavelength at 3.4 mW/mm2 power density of the laser beam have been mentioned in Figure 5h and are obtained as 1.53 and 0.74 s, respectively. Similarly, we can measure the response time and recovery time of the photodetector when it has been irradiated by wavelengths of 635 and 785 nm at 3.4 mW/mm2 power density. The response time obtained at 635 nm is found to be 1.06 whereas that at 785 nm is 1.18 s, and the recovery time obtained at 635 nm is found to be 0.53 s whereas that at 785 nm is 0.63 s To further analyze the performance of the fabricated photodetector, the experiments were performed in vacuum as well as in air. Since, on illumination of the photodetector with 1064 nm wavelength and 3.4 mW/mm2 power density of the laser beam, the performance of the photodetector is found to be optimized, here, the experiments were also performed with the same parameters of the laser beam. The variation of the output current as a function of time under vacuum and in air is depicted in Figure 6a. It is clearly visible from the figure that the photocurrent of the device measured in vacuum was 12.25 μA whereas in air, it was 7.8 μA. The photocurrent in air was found to be about 1.5 times lower than that in vacuum. The reason for the lower photocurrent in air condition can be explained using the pictorial representation of Figure 6b. The electrons are dominant carriers in n-type TiS3 nanoribbons. Under illumination of the photodetector, TiS3 nanoribbons absorb photons and generate electron−hole pairs. In air, oxygen molecules also get adsorbed onto the nanoribbons’ surface that captures the free electron from the nanoribbons, resulting in the disappearance of free electrons, which decreases the concentration of electrons in the nanoribbons’ surface, which in turn decreases the photocurrent and hence the conductivity. Contrary to this, when the device was illuminated with the same wavelength and power density of the laser beam under vacuum condition, oxygen adsorption was negligible. Therefore, the concentration of free electrons was higher in vacuum than in air, which shows increase in the conductivity of the photodetector in vacuum. This finding supports the trapping mechanism in the n-type semiconductor photodetectors. Moreover, the additional advantage of the present liquidphase-aligned TiS3 nanoribbon photodetector over the solidstate TiS3-based photodetector is that it is a simple, low cost, robust, and easy method of fabrication of the device, which can be easily scaled up for industrial applications. On the other

Figure 6. (a) Variation of the output current as a function of time for the fabricated photodetector under illumination from 1064 nm wavelength and 3.4 mW/mm2 power density measured in vacuum and in air, (b) pictorial representation showing the absorption/ desorption behavior.

hand, the solid-state fabrication technique involves an expensive and complex photolithography technique, which involves huge cost and man power.

4. CONCLUSIONS In summary, fabrication and characterization of the horizontally aligned titanium tri sulfide (TiS3) nanoribbon-based photodetector have been carried out. We have fabricated the photodetector by horizontally aligned TiS3 nanoribbons in between gold interdigitated electrodes (IDE) onto the Si/SiO2 substrate. The fabrication methods involve micro-electromechanical systems, photolithography, and dielectrophoresis (DEP). First of all, structural and morphological properties of the TiS3 nanoribbons were investigated using XRD, SEM, TEM, UV−vis, and EDS characterization techniques. Then, photodetector-based characteristics were investigated by illuminating the device with a fiber-coupled laser beam in the visible-to-NIR region of the electromagnetic spectrum. The photodetector characteristics were optimized by varying wavelength and power density of the laser beam. It has been well revealed that the fabricated photodetector possesses an extremely low dark current of 2.51 μA and a sufficiently large ON/OFF ratio of about 5.01 on illuminating with a 1064 nm wavelength laser beam at 3.4 mW/cm2 power density. The photodetector demonstrated reasonably good responsivity, quantum efficiency, and detectivity. The maximum responsiv6188


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(6) Abid; Sehrawat, P.; Islam, S. S.; Gulati, P.; Talib, M.; Mishra, P.; Khanuja, M. Development of highly sensitive optical sensor from carbon nanotube-alumina nanocomposite free-standing films: CNTs loading dependence sensor performance Analysis. Sens. Actuators, A 2018, 269, 62−69. (7) Misse, P.; Berthebaud, D.; Lebedev, O.; Maignan, A.; Guilmeau, E. Synthesis and Thermoelectric Properties in the 2D Ti1−xNbxS3 Trichalcogenides. Materials 2015, 8, 2514−2522. (8) Iyikanat, F.; Sahin, H.; Senger, R. T.; Peeters, F. M. Vacancy Formation and Oxidation Characteristics of Single Layer TiS3. J. Phys. Chem. C 2015, 119, 10709−10715. (9) Abid; Sehrawat, P.; Islam, S. S.; Mishra, P.; Ahmad, S. Reduced graphene oxide (rGO) based wideband optical sensor and the role of Temperature, Defect States and Quantum Efficiency. Sci. Rep. 2018, 8, No. 3537. (10) Barawi, M.; Flores, E.; Ponthieu, M.; Ares, J. R.; Cuevas, F.; Leardini, F. Hydrogen storage by Titanium based sulfides: nanoribbons (TiS3) and nanoplates (TiS2). J. Electr. Eng. 2015, 3, 24−29. (11) Yang, W.; Hu, K.; Teng, F.; Weng, J.; Zhang, Y.; Fang, X. HighPerformance Silicon-Compatible Large-Area UV-to-Visible Broadband Photodetector Based on Integrated Lattice-Matched Type II Se/ n-Si Heterojunctions. Nano Lett. 2018, 18, 4697−4703. (12) Mishra, M.; Gundimeda, A.; Krishna, S.; Aggarwal, N.; Gahtori, L. G. B.; Bhattacharyya, B.; Husale, S.; Gupta, G.; et al. SurfaceEngineered Nanostructure-Based Efficient Nonpolar GaN Ultraviolet Photodetectors. ACS Omega 2018, 3, 2304−2311. (13) Fang, X.; Bando, Y.; Liao, M.; Zhai, T.; Gautam, U. K.; Li, L.; Koide, Y.; Golberg, D. An Efficient Way to Assemble ZnS Nanobelts as Ultraviolet-Light Sensors with Enhanced Photocurrent and Stability. Adv. Funct. Mater. 2010, 20, 500−508. (14) Liu, S.; Xiao, W.; Zhong, M.; Pan, L.; Wang, X.; Deng, H.-X.; Liu, J.; Li, J.; Wei, Z. Highly polarization sensitive photodetectors based on quasi-1D titanium trisulfide (TiS3). Nanotechnology 2018, 29, No. 184002. (15) Liu, G.; Lia, Z.; Chen, X.; Zheng, W.; Feng, W.; Dai, M.; Jia, D.; Zhou, Y.; Hu, P. A. Non-Planar Vertical Photodetectors Based on Free-Standing Two-Dimensional SnS2 Nanosheets. Nanoscale 2017, 9, 9167−9174. (16) Wang, L.; Shao, Z.; Zhang, Q.; Zhang, X.; Wang, Y.; Sun, Z.; Lee, S. T.; et al. MoS 2 /Si Heterojunction with Vertically Standing Layered Structure for Ultrafast, High-Detectivity, Self-Driven Visible− Near Infrared Photodetectors. Adv. Funct. Mater. 2015, 25, 2910− 2919. (17) Zheng, B.; Chen, Y.; Wang, Z.; Huang, F. Q. Z.; Hao, X.; Li, P.; Zhang, W.; Li, Y.; et al. Vertically oriented few-layered HfS2 nanosheets: growth mechanism and optical properties. 2D Mater. 2016, 3, No. 035024. (18) Niu, Y.; Frisenda, R.; Flores, E.; Ares, J. R.; Jiao, W.; Lara, D. P.; Sánchez, C.; Wang, R.; Ferrer, I. J.; Gomez, A. C. PolarizationSensitive and Broadband Photodetection Based on a MixedDimensionality TiS3/Si p−n Junction. Adv. Opt. Mater. 2018, 6, No. 1800351. (19) Rahmati, B.; Hajzadeh, I.; Karimzadeh, R.; Mohseni, S. M. Facile, scalable and transfer free vertical-MoS2 nanostructures grown on Au/ SiO2 patterned electrode for photodetector application. Appl. Surf. Sci. 2018, 455, 876−882. (20) Ning, Y.; Zhang, Z.; Teng, F.; Fang, X. Novel Transparent and Self-Powered UV Photodetector Based on Crossed ZnO Nanofiber Array Homojunction. Small 2018, 14, No. 1703754. (21) Novoselov, K. S.; Geim, A. K.; Morozov, S.; Jiang, D.; Zhang, Y.; Dubonos, S.; Grigorieva, I.; Firsov, A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (22) Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, No. 136805. (23) Novoselov, K.; Jiang, D.; Schedin, F.; Booth, T.; Khotkevich, V.; Morozov, S.; Geim, A. K. Two-dimensional atomic crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10451−10453.

ity, quantum efficiency, and detectivity were estimated to be 5.22 × 102 A/W, 6.08 × 102, and 1.69 × 109 Jones, respectively, which are obtained at 1064 nm wavelength and 0.62 mW/mm2 power density of the laser beam. From the control experiments, it is found that the photoresponse of the horizontally aligned TiS3 nanoribbons is better than that of the randomly oriented TiS3 nanoribbon-based photodetector. Finally, the performance of the present photodetector was compared to that of the reported ones and found to outperform that of the reported ones. In totality, the results suggest that the TiS3 nanoribbons are promising for nanoscale electronics and optoelectronic devices. We believe that this research will allow the recognition of high-performance TiS3 nanoribbons and will play a positive role in nanodevices with 1D transition-metal trichalcogenide nanostructures.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b03067. UV−vis absorbance of the fabricated TiS3 nanoribbons, variation of output current for (a) 635 nm, (b) 785 nm, and (c) 1064 nm wavelength of the laser beam as a function of time for the randomly oriented TiS3 nanoribbons, variation of output current for the 635 nm wavelength of the laser beam as a function of time for increased number of cycles and magnified images of some cycles (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 011-2698-1717 ext. 3229. ORCID

Prabhash Mishra: 0000-0001-9586-6939 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We greatly appreciate the financial support from Science and Engineering research board (SERB), DST India, project no. ECR/2017/000530.

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REFERENCES

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