Wavelength dependent anisotropic photosensing activity of zirconium trisulfide crystal

According to the performance requirements, either bulk or nanocrystalline form of the material can be used for different types of device applications. In the present study, zirconium triselenide bulk crystals were grown by direct vapour transport technique. The energy-dispersive X-ray analysis (EDAX) confirms the purity of the grown crystals. The as-grown crystals and powder have been examined under Carl Zeiss optical microscope and scanning electron microscope (SEM) for morphological studies which revealed the evolution of crystalline phases of the material by the layered kind of growth mechanism. The transmission electron microscopy (TEM) with selected area electron diffraction (SAED) pattern analysis showed the grown crystals possess good crystallinity, whereas X-ray diffraction (XRD) analysis confirmed the monoclinic phase of the crystals. To study the effect of different wavelength sources (Blue-470 nm, Green-540 nm, Red-670 nm) on bulk zirconium trisulfide photodetectors, a pulse photo response experiment was carried out. The anisotropic behaviour is also revealed using the same sources. Various device parameters like responsivity, sensitivity, detectivity and external quantum efficiency (EQE%) were calculated. The highest responsivity and detectivity of 81.7 µA/W and 3.56 × 107 Jones were achieved for blue (470 nm) light source, respectively.


Introduction
Two-dimensional (2D) layered materials have emerged as a new class of materials with unusual optical, electrical, mechanical and thermal properties [1][2][3]. Owing to their unique properties, they attracted researcher's attention for applications in energy conversion, flexible electronics and information technology fields [4][5][6]. Transition Metal Trichalcogenides (TMTCs) of layered group IV with the chemical formula of MX 3 , e.g. M = Zr, Ti and X = S, Se, Te, are known to be anisotropic 2D systems [7,8]. These trichalcogenides are often synthesized in thin fibrous ribbon shape and offer several interesting phenomena originating from their large structural inplane anisotropy as individual MX 3 layers are made out of moderately interacting 1D-like chain structures [9][10][11][12]. Amongst these materials, particularly, transition metal trisulfides (TMTS) are receiving the attention of researchers because of the number of inherent properties. As the groups (S-S) 2have the capability of reversibly accepting a pair of electrons [13], these species are studied as cathode materials in chemical current sources [14], including ZrS 3 for primary thermal Li battery as well as semiconductor TiS 3 [15]. Some transition metal polysulfides, in particular semiconducting MS 3 , are of interest as catalysts in the production of oxygen (ZrS 3 ) [16] or as gas sensors (TaS 3 ) [17]. Zirconium trisulfide (ZrS 3 ) attracted interest owing to the theoretically calculated 1.79 eV bandgap value for bulk ZrS 3 [18]. Recently, light sensors based on ZrS 3 material were fabricated using the nanoribbons and nanobelts forms and tested under various illumination sources like LASER [19], white light [20] and Ultraviolet to NIR [21] and was reported. These devices showed good responsivity towards respective illuminations with minimum external bias. Despite these recent researches, this light sensing capacity of the bulk crystalline material has not yet been studied. Also, the light sensing activity of both in-plane and out-of-plane symmetries is not yet revealed. In the present study, the growth process, characterization and application of ZrS 3 material in its bulk crystalline form are demonstrated.

Growth of ZrS 3 crystal
The bulk ZrS 3 crystals were grown by direct vapour transport technique. Initially, the inner surface of a quartz ampoule having dimensions of 24 cm length, 2.2 cm inner diameter and 2.4 cm outer diameter was cleaned with sulfuric acid, hydrochloric acid and simple water periodically one by one. The bottom part of the ampoule was etched by evaporating hydrofluoric acid at 100°C temperature to create enough rough surface that provides nucleation points. Further, the ampoule was cleaned with simple water and acetone and set to dry in the oven at 200°C. This highly cleaned ampoule was then filled with pure zirconium (99.99%) and sulfur (99.99%) powder with stoichiometric proportions and sealed at a pressure better than 10 -5 Torr. This sealed ampoule was then inserted into a high-temperature dual-zone horizontal furnace. The ampoule was set in such a way that the half part is situated in higher temperature zone and the other half part in lower temperature zone. The temperature of the furnace was initially raised to 850°C for 40 h, the reaction time was set for 60 h at maximum temperature and in the last stage, the furnace was cooled down to room temperature in 80 h with slow temperature decrement. The temperature difference between the two zones was maintained at 50°C throughout the cycle. The process yielded needle-shaped thin flakes with a shiny metallic appearance.

Characterization
The as-grown compound was analysed quantitatively by performing energy-dispersive X-ray analysis (EDAX) with elemental mapping. The scanning electron microscopy (SEM) of the as-grown ZrS 3 compound was carried out. Both the EDAX and SEM analyses were performed using FEG-SEM 450. The grown crystal was analysed under the Carl Zeiss optical microscope to study the nature of the surface of the as-grown ZrS 3 crystal. The morphological and structural analysis of the grown crystalline ZrS 3 compound was done by transmission electron microscopy (TEM) with selected area electron diffraction (SAED) pattern using TEM Tecnai 20 (Philips, Holland) and powder X-ray diffraction (PXRD) 2.2 kW Cu (ka) incident radiation and Rigaku Ultima IV powder X-ray diffractometer, respectively.
Further, the grown single crystal was characterized for the study of the photo-sensing activity. To study the ability and efficiency of ZrS 3 single crystal, it was analysed under the Carl Zeiss optical microscope, and the smooth surface essential for the fabrication of good quality photodetector can be observed. To fabricate the photodetector, a shiny needle-shaped ZrS 3 crystal having an area of 0.04 cm 2 was cleaved using scotch tape to get a fresh and clean surface. This crystal was then attached to a piece of mica sheet (1 9 1 cm 2 ). Two copper wires were attached at two ends of the crystal to get in-plane symmetry (parallel to the basal plane) and one was attached at the bottom side of the crystal to get out-of-plane symmetry (conduction perpendicular to the basal plane) using conductive silver paste (Fig. 1). This mica sheet was further attached on a Printed Circuit Board (PCB) which was connected to Keithley 4200-SCS. The schematic diagram of the prepared device and connections is shown in Fig. 1. A pulse photo response experiment was carried out to study the switching behaviour of ZrS 3 crystal towards three different wavelength sources (Blue-470 nm, Green-540, Red-670 nm) for inplane and out-of-plane contact symmetries.

Material characterization
In the growth process, the temperature was raised gradually in the first cycle to turn the constituent elements into vapour form steadily from the pure metal powder. The second cycle was set for a constant temperature value so that these vaporized materials react with each other properly. In the last cycle, the furnace was cooled down to room temperature such that the material gets condensed in a crystalline form based on the nucleation points created at the lower end of the ampoule where the etching was done. In the end, fine shiny metallic crystals were collected. The elemental investigation of ZrS 3 was carried out using EDAX. Table 1 shows the calculated and observed stoichiometric proportion of the constituent elements, i.e. zirconium and sulfur. Figure 2 shows the uniform distribution of Zr and S throughout the area covered by the as-grown crystal. The observed values are near to the calculated values and the final product is zirconium rich material as per the EDAX data.
The few fresh as-grown crystals were then selected for the study under the optical microscope. The optical image of the as-grown ZrS 3 crystals having dimensions in few micrometres, as well as the layers produced during the growth process, is shown in Fig. 3a and b. In Fig. 3a, the edge of a crystal is shown in which the stacking of layers is observed. Figure 3b shows the top view of the crystal in which layers are produced with a pointed end. The vertically stacked layers depict that the grown material has a multilayer structure which is suitable for anisotropic studies [22]. Figure 3c and d shows the SEM image of the asgrown compound. The grown compound contains strips having length and width in the micrometre range. These strips are nothing but well-grown microcrystals of ZrS 3 materials. In Fig. 3d, a monoclinic shaped object is found. In many previously published research articles, it is claimed that ZrS 3 has a monoclinic crystal structure. So, this object found in the SEM image indicates in a confirmative way that the grown material has a monoclinic structure.
Transmission Electron Microscopy (TEM), Selected Area Electron Diffraction (SAED) pattern and X-ray Diffraction (XRD) further established the structural confirmation of the material. The crystalline ZrS 3 compound was added into acetone and sonicated for around 1 h. One drop of such sonochemically exfoliated material was put on a copper grid, and the  TEM analysis was performed. Figure 4a is the TEM image of crystalline ZrS 3 nanoflake which has a contrasting appearance due to the thickness of the crystal, which is shaped in a strip form. The dark vicinity of the flake is due to its higher thickness. The SAED pattern is likewise recorded for the nanoflake sample using a 200-kV electron beam. The single crystalline nature can be confirmed from the bright spot pattern, as witnessed in Fig. 4b. The ZrS 3 material was further characterized by X-ray Diffraction (XRD). The XRD pattern is wellindexed with the monoclinic structure as shown in Figure S1 (Supplementary Information). ZrS 3 possesses a monoclinic structure with crystallographic parameters: a = 5.124 Å , b = 3.624 Å , c = 8.980 Å and b = 97.28°, which are in good agreement with reported values [18][19][20]. Moreover, the sharpness of the prominent peaks shows the good crystalline nature of the grown material. The absence of peaks corresponding to other compounds confirms the singularity of phase in the grown material.

Current-time characteristics
TMTC [23][24][25] and TMDC [26][27][28][29] materials are layered materials that lead to the orientation-dependent material properties in different directions. Due to the stacking of layers, these TMC materials provide anisotropy [19] [32]. Owing to the previously published articles for anisotropy in layered materials, ZrS 3 crystal is also expected to display strong anisotropy. We, therefore, investigated the anisotropic photoresponse of the ZrS 3 crystal. To study the anisotropy of grown ZrS 3 crystal in terms of current-voltage (I-V) and current-time (I-t) Fig. 2 Elemental spectrum for the as-grown ZrS 3 crystal, the exposed area of the material for elemental mapping and uniform distribution of all the elements characteristics, two types of contacts were taken: (1) conduction along the basal plane (? to c-axis) and (2) conduction perpendicular to the basal plane (k to c-axis) as shown in Fig. 1. The corresponding (I-V) curves for both configurations are shown in Fig. 4a and c, respectively. The I-V characteristics of ZrS 3 crystal were carried out under the dark and three monochromatic illumination sources, i.e. blue (470 nm), green (540 nm) and red (670 nm) LEDs having power 100 mW/cm 2 at 1 V external bias using Keithley 4200-SCS. For that, copper electrodes were attached to the crystals with the help of conductive silver paste. The I-V characteristics of the ZrS 3 crystal under dark and illumination conditions are shown in Fig. 4a and c for both types of contact configurations. It can be inferred from the graph that the ZrS 3 device shows good ohmic nature for similar contacts. In both contact configurations, the current increased upon illuminating the device by different lights, indicating good photoresponse of the material. Besides, it is observed that the current along the direction parallel to the basal plane is higher than perpendicular to the basal plane. The dark current along the parallel direction to the basal plane is 447.7 nA, which is larger than 61.15 nA in the direction of contacts perpendicular to the basal plane. The device was illuminated by the blue (470 nm) light, and the current rises to 0.448 lA in contacts ? to c-axis, whereas the current rises to 0.061 lA in contacts k to c-axis. These values of current inferred the strong anisotropy of ZrS 3 . This anisotropy is also demonstrated well in I-t plots as shown in Fig. 4b and d. Chauhan et al. described the effect on the current flow mechanism in these kinds of symmetric (? to c-axis) and anti-symmetric contact (k to c-axis) configurations by demonstrating a model [22]. In consideration of contact configuration ? to c-axis (ab-plane), few upper layers of the multilayer crystal are affected by the light source according to the depth of absorption of incident photons and bias voltage. These are the only layers which may get involved in the dominant conduction. These layers have zirconium and sulfur atoms bonded with covalent bond along with the basal plane. Due to the external bias carrier conduction occuring in this case is similar to the case of a planer npn or pnp transistor structure. The charge carriers require less amount of external energy in the case of these few layers bonded by covalent bond as the less amount of resistance faced by the carriers. As we can observe in the optical and SEM images, ZrS 3 crystal has a multilayer structure in which number of layers are stacked one upon the other and remains bonded with each other by a weak van der Waals bond. This kind of bonding needs large amount of external energy when carriers have to flow perpendicular to the basal plane, i.e. contact configuration k to c-axis. In this case, all the layers of the crystal would take part in carrier transportation perpendicular to the contacts in the direction of ab-plane (basal plane). The time carriers would face more resistance crystal under different illuminating wavelengths for contacts along the basal plane (? to c-axis) and contacts parallel to the c-axis, respectively c Fig. 6 Detector parameters a, b, c, d, e for contacts along the basal plane (? to c-axis) and g, h, i, j, k for contacts perpendicular to the basal plane (k to c-axis) compared to the case of contact configuration ? to c-axis which leads to the fall in the photocurrent.
The detector parameters such as responsivity (R k ), specific detectivity (D), sensitivity (S) and external quantum efficiency (EQE%) are evaluated for ZrS 3 using the following equations (Eqs. 1-4): Here I ph (I ph = I ill -I dark ; I ill = current under illuminated condition and I dark = current under dark condition) is the photocurrent, P is the illumination intensity, A is the effective area of the photodetector, h is Planck's constant, c is the speed of light in vacuum, k is the wavelength of incident radiation, e is the elementary electronic charge (1.6 9 10 19 C) and I dark is the dark current. The I-t characteristics of the device were carried using the same sources for both contact configurations at 1 V bias voltage which is shown in Fig. 5b and d. For both contact configurations, the current rises with the illumination of light. The highest photocurrent noted was 245.7 nA for the contacts ? to c-axis and 33.54 nA for the contacts k to c-axis in the blue (470 nm) light illumination. This photocurrent reduced to 166.22 nA for the contacts ? to c-axis and 19.33 nA for the contacts k to c-axis in the red (670 nm) light illumination.
Similar decrements are also observed in the detector parameters as shown in Fig. 6. The graphs of the detector parameters, i.e. photocurrent, sensitivity, responsivity, detectivity and EQE (%) against wavelength are plotted. The highest responsivity and detectivity of 81.7 lA/W and 3.56 9 10 7 Jones were achieved for blue (470 nm) colour light, respectively. It can be noted clearly from the scales of all plots that the device shows significant anisotropy and with the increment of incident light wavelength, all the detector parameters are getting decreased for both types of contact configurations. So, the ZrS 3 material in bulk crystal form possesses a good ability to be used in the preparation of the photo-sensing device having anisotropic behaviour. Table 2 shows the comparison of detector parameters of ZrS 3 -based photodetectors with previously reported detectors.

Conclusion
In summary, the ZrS 3 single crystals are successfully synthesized by the direct vapour transport technique. The EDAX characterization confirms that no impurity is found in the grown material, and the mapping depicts the uniform distribution of the all constituent elements. The optical images and SEM images show that the grown material has a layered kind of growth which leads to the uniqueness in the behaviour of many detector parameters. TEM, SAED and XRD analyses showed that the crystals possess good crystallinity. From the study of current-voltage (I-V) and current-time (I-t) characteristics, it is clear that the ZrS 3 material possesses strong anisotropy as the device showed good performance for contact configuration made along the basal plane (? to c-axis) compared to the contact configuration made perpendicular to the basal plane (k to c-axis). Due to this uniqueness, the material has good candidature for its application in the field of optoelectronic device applications.