Impact of Different Morphological Structures on Physical Properties of

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C: Physical Processes in Nanomaterials and Nanostructures

Impact of Different Morphological Structures on Physical Properties of Nanostructured SnSe Gowthamaraju Shanmugam, Uday P. Deshpande, Alfa Sharma, Parasharam M. Shirage, and Preeti A. Bhobe J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03797 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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The Journal of Physical Chemistry

Impact of Different Morphological Structures on Physical Properties of Nanostructured SnSe Gowthamaraju Shanmugam,† U. P. Deshpande,‡ Alfa Sharma,¶ P. M. Shirage,¶,† and P. A. Bhobe∗,†,¶ †Discipline of Physics, Indian Institute of Technology Indore, Khandwa Road, Simrol, Indore-453 552, India ‡UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452 017, India ¶Department of Metallurgy and Materials Science, Indian Institute of Technology Indore, Khandwa Road, Simrol, Indore-453 552, India E-mail: [email protected]

Abstract Nanostructured SnSe samples with specific morphologies like rods, rod-flowers, flakes, and flake-flowers were synthesized using the same reactants, but with a control over various growth parameters involved in a hydrothermal reaction process. Morphology and detailed micro-structure of obtained samples has been studied by means of field-emission scanning electron microscopy (FE SEM) and high resolution transmission electron microscopy (HR-TEM). The sample quality, homogeneity, and phase purity, has been thoroughly studied by selective area electron diffraction (SAED) and X-ray diffraction (XRD) analysis. Crystal structure study by refinement of powder XRD profiles using least squares fitting strategy indicates all samples to be pure single phase and provides its lattice parameters. The impact of changing morphology on the

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electrical, optical and sensing properties of SnSe has been systematically evaluated. Temperature dependent electrical transport measurement highlight the increasingly 2D nature of the nano-flakes, while lowest resistivity is obtained for nano-rods. The mechanism for charge transport shows a crossover from thermally activated band conduction to a Coulomb interaction induced localization of charge carriers. Optical gap energy and absorbance profile also shows morphology dependent response, as monitored using ultraviolet−visible−near-infrared (UV-Vis-NIR) spectroscopy. Resistive sensors for humidity sensing application show quite high sensitivity (∼ 1500 %), good repeatability, low hysteresis in absorption-desorption process and good reproducibility at room temperature. Amongst different SnSe samples, the rod morphology gives the best sensing performance.

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Introduction

Tin selenide (SnSe) is a typical IV-VI group binary layered p-type semiconductor 1 that belongs to the metal chalcogenides family (MX, M=metal and X=S, Se, Te). It has a tunable band gap 2 (direct 1.3 eV and indirect 0.9 eV) with anisotropic transport property 3 that attracts equal attention of researchers working in diverse areas. The anisotropy arises out of its orthorhombic structure 4 with Pnma space group. The arrangement of atoms is often described as a zigzag double layer of Se2− and Sn2+ with strong intra layer bonds and a weak van der Waals interaction between the layers. A change in symmetry from Pnma to Cmcm occurs at 800 K. 5 Amongst its many applications such as super-capacitor, 6 photovoltaic, 7 thermistor and photoresistor, 8 phase change memory application, 9 solar cells 10 etc, the most prominent is the unprecedented thermoelectric ZT value of ≈ 2.6. 11 Besides, its non-toxic, chemically stable, and, environmental friendly factors makes it a grandeur multifunctional material. Nanotechnology offers an extra handle to materials scientists to innovative and tune the properties of technologically relevant materials. There are innumerable examples in litera-

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ture demonstrating the ability to design novel functional devices by control over size and morphology of nanometer- and micrometer-sized semiconductors. 12 Bringing down the dimensionality of a layered semiconductor by tuning its morphology in addition to its size leads to spatial confinement effect arising out of domination of electromagnetic force that get modified at the corners, edges and crystallographic faces of the morphological structure. A lot of effort has been made to prepare low dimensional SnSe structures using spray pyrolysis, 13 pulsed laser deposition, 14 brush plating, 15 flash evaporation, 16 atomic layer deposition, 17 hot wall epitaxial, 18 chemical vapour deposition, 19 electrodeposition, 20 chemical bath deposition, 21 arc-melting, 22 hydrothermal method 23 etc. No doubt, every such method has its own pros and cons and often the trade-off is set between increasing chemical toxicity, environmental hazards and low cost production. Amongst these plethora of approaches, solution-based hydrothermal synthesis offers a promising alternative due to its low growth temperature, low cost, and high precision control over all the synthesis parameters, to provide desired product. The protected reaction environment of an autoclave gives highly pure product without employing any organometallic precursor or toxic gases and the super-critical stage of the reaction is easily attainable under high pressure. There are many reports in literature that discuss the hydrothermal synthesis of SnSe and associated modification in its properties that can be exploited for technological applications. To list a few here, Ge et al., 24 discussed low temperature transport of SnSe nanorods for thermoelectric application, Ramasamy et al., 25 reported preparation of hierarchical nanostructured SnSe and discussed its behavior as an electrode for dye sensitized solar cells. Pawbake et al., 26 investigated the humidity sensing performance of SnSe nanorods. Each of these reports present in literature discuss the specific preparation procedure adopted by them and the corresponding enhancement in certain physical property of interest. It may be noted that the nanostructured growth mechanism leading to control of specific morphology is still a challenging factor and has hitherto not been reported. A successful control over the morphology of nanostructured SnSe, especially using the

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same precursor solution and the ability to simultaneously tune its optical, electrical and sensing properties, will take us to the next level of targeted synthesis coupled to practical applications. In this paper, we present a successful attempt of control over the morphology of nanostructured SnSe. In particular, we study the impact of different morphological structures like rods (R), rod-flowers (RF), flakes (F), flake-flowers (FF), on physical properties of nanostructured SnSe, obtained by controlling the growth parameters of a simple hydrothermal reaction. The novelty in our approach lies in using the same starting reactants to obtain targeted morphologies of SnSe. After confirming the morphology using electron microscopy, the crystal structure and growth orientation was studied using powder XRD and high resolution transmission electron microscopy. These phase pure, stoichiometric compositions were further studied for its electrical transport property in the temperature range of 100 to 300 K. Detailed analysis of the temperature dependence of resistivity reveals that the conduction mechanism is changes from thermally activated band conduction at high temperature regime to Efros-Shklovskii variable range hopping (ES-VRH) mechanism at low temperature regime, where electron correlation effect begin to gain significance. Optical gap energy also changes with the changing morphology, and variation in optical absorbance and sharpness of transition indicate formation of in-gap states. Resistive sensors for humidity sensing application show quite high sensitivity (of the order of 1500 %), good repeatability, low hysteresis and good reproducibility at room temperature. The performance of rod shaped particles is superior to rest of the morphologies studied here. In the end, thermal stability of synthesized nanostructures was also confirmed using the thermal gravimetric analysis.

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Experimental Details

For the typical hydrothermal method of synthesis adopted here, all the chemicals used were of the analytical grade with purity 99.99% or higher. Deionized water (DI) of the order of MΩ-cm resistivity was used for preparation of aqueous solutions. All the chemicals were

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purchased from Alfa Aesar and used without any further purification. In a typical growth procedure, SnCl2 . 2H2 O and SeO2 were used as source of Sn and Se, respectively. Hydrazine hydrate (5ml) was added to SeO2 /EG (ethylene glycol) solution under continuous stirring condition, while sodium hydroxide (NaOH) was added to SnCl2 /DI solution. The resulting solutions of SnCl2 and SeO2 were slowly mixed and stirred for 30 min. The homogeneous solution thus obtained was transferred to a 50 ml Teflon-lined autoclave and heated at desired temperature and duration, as per the details mentioned in Table 1. The final SnSe product in powder form was obtained from centrifugation of the heat treated solution. This powder was washed several times with DI water and ethanol solution before leaving it to dry under vacuum. Table 1: Synthesis parameters corresponding to different morphology of SnSe. RT refers to room temperature; Precursor conc. refers to the concentration of SnCl2 . 2H2 O and SeO2 . Precursor Reaction Duration ◦ conc. Temperature ( C) (hr) 1.0 g RT 03 1.0 g 170 03 0.5 g 170 03 1.0 g 170 12

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Morphology Rod Flowers (RF) Rods (R) Flake Flowers (FF) Flakes (F)

Results and Discussion

Morphology of all the SnSe samples was analyzed by a field-emission scanning electron microscope (FE-SEM ; ZEISS Gemini supra 55). The detailed microstructure and growth orientation was examined using high resolution transmission electron microscope (HRTEM; JEOL, 2100F model, 200 kV). We obtained a total of four different SnSe morphologies, viz. rods, flakes, rods grown into flower forms (rod-flowers), and, flakes grown into flower form (flake-flowers) from the same starting solution described in the preceding section. Each morphology developed solely by systematic control over growth parameters. The repeatability of the process in yielding the respective morphology was tested several times and found 5



satisfactory. The reaction mechanism in formation of SnSe is known to follow the ion-ion combination pathway, as described in great detail by C. Wang et al. in Ref. 27 A concise mechanism of such a reaction is presented here: SnCl2 · 2 H2 O −−→ Sn2+ + 2 Cl – + 2 H2 O SeO2 + N2 H4 −−→ Se ↓ + N2 + 2 H2 O Sn2+ + Se −−→ Se2 – + Sn4+ Sn2+ + Se2 – −−→ SnSe ↓ Using hydrazine hydrate alone under mild alkaline reaction conditions, SnSe can be synthesized at considerably low temperature of 70 ◦ C. 27 In addition, use of NaOH not only provides the high alkaline environment conducive for the reaction, but substantially diminishes the formation of SnO and SnO2 phases by engaging the extra Sn2+ ions in the solution. 24 Thus, during the current synthesis procedure, the final homogeneous solution when allowed to rest in non-oxidizing environment of the sealed autoclave provides requisite temperature from the combustive nature of hydrazine hydrate EG combination, and pressure from gases released during reaction, to yield SnSe crystallites. Further, the large difference of electronegativity of Se and Sn atoms may be inducing a strong polarity to the (00l ) planes of the freshly crystallized particles. The polar EG molecules present in the solution tend to get adsorbed on such charged crystal planes through electrostatic interactions, inhibiting the growth of the crystallites along these planes. Therefore, the crystallite starts to grow along other general direction like the (111) planes. More than one crystallite is held together by the Columbic interaction between the SnSe planes and EG molecules, while unhindered growth of crystallites takes place along the (111) direction. The morphology of such a sample develops to be of nano-rods arranged in the form of ‘petals’, attached at one end and freely protrude out at the other end (see Figure 1). This gives it an appearance of 2 µm sized rod-flower (RF). Each of the petals of this rod-flower is 400 nm long and has a diameter of 60 nm. Further, the analysis of its HRTEM image and associated SAED pattern shown in

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Figure 1c & d, yields inter-planer distances 2.9 and 3.2 ˚ A, in good agreement with predominant peaks of XRD pattern. Other specific morphologies develop upon heat treatment of the solution and changing the precursor concentration, as summarized in Table 1.

Figure 1: (Color online) (a) and (b) Low(2 µm) and high(400 nm) magnification FE-SEM images, (c) HRTEM image(5 nm) and (d) SAED pattern obtained for nanostructured SnSe with rod-flower (RF) morphology. Next, the rods (R) morphology is obtained by heat treating the same homogeneous solution that yielded rod - flowers. As this solution is heated at 170 ◦ C for a period of 3 hr, the crystallites that were originally in the form of ‘petals’ start growing in size. Also, the raised temperature of the solution provides enough energy to overcome the electrostatic interactions with EG and the ‘petals break free into individual rods. Given that the unit cell structure of SnSe is a orthorhombic crystal with the axial parameter (along a-axis) twice as large as the basal parameters, the length of the petals grow much more in length as compared to its width. The detached individual rods (R) of length 500 nm and diameter 100 nm, as seen from the corresponding FE-SEM micrographs presented in Figure 2a & b. A d-spacing of 0.32 ˚ A can be extracted from its HRTEM image presented in Figure 2c, whereas, well defined arrangement of spots in the SAED image (Figure 2d) indicate the highly oriented single crystalline nature of the sample. 7



Figure 2: (Color online) (a) and (b) Low(2 µm) and high(400 nm) magnification FE-SEM images, (c) HRTEM image(5 nm) and (d) SAED pattern obtained for nanostructured SnSe with rod (R) morphology. Instead of limiting the heating time to 3 hr, if the above homogenous solution is continued to be heated at 170◦ C for a period of 12 hr, we observe the formation of individual nanoflakes (F). A prolonged heat treatment increases the reaction time leading to the coalescence of the already formed nano-rods along the a-axis direction. This process leads to formation of large platelets or flakes morphology with a preferential orientation of (400) planes, as also confirmed from its XRD pattern discussed later in the text. Here, Na+ ions from NaOH acts as a mineralizing agent that further enables the formation of large flakes. 28 Such flakes have an average thickness of 70 nm as shown in its FE-SEM micrographs at Figure 3a & b. The thickness of these flakes is much smaller than its lateral size emulating a 2D planer nature of bulk SnSe. HRTEM image and SAED pattern (Figure 3c & d) shows d spacing of 3.3 nm with corresponding growth direction of (400) plane. Analogous to the rod-flowers, the flake-flower (FF) morphology can also be obtained from the same synthesis upon reducing the concentration of the initial precursors (SnCl2 . 2H2 O and SeO2 ) by half (see Table 1). The same chemical reaction as described above comes into play, but with significantly reduced nucleation sites. Again, EG docks these few nuclei at its 8


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Figure 3: (Color online) (a) and (b) Low(2 µm) and high(400 nm) magnification FE-SEM images, (c) HRTEM image(5 nm) and (d) SAED pattern obtained for nanostructured SnSe with flake (F) morphology. polar heads and the subsequent heat treatment leads to the growth of the grains, albeit, in an oriented fashion with reduced particle interfaces. Also, lower nucleation density implies an increase in the relative content of the mineralizing agent, NaOH, further supporting the growth of large grains. This entire process culminates in the formation of large flakes, attached at one end in a flower-like fashion. As can be seen from the FE-SEM images in Figure 4a & b, these flake-flowers have an average size of 4 - 5 nm. The basal size in terms of each petal’s width is much larger as compared to its thickness of 10 nm. HRTEM and SAED images (Figure 4c & d) are in congruence with the RF and R samples. Each sample powder was nicely packed on the glass substrate and XRD profiles were recorded using Rigaku SMARTLAB instrument equipped with monochromatic Cu Kα (λ = 1.541 ˚ A) radiation. Profiles were recorded in the range, 20◦ ≤ 2θ ≤ 60◦ , as shown in Figure 5a. All the observed peaks can be indexed to orthorhombic crystal structure with P nma space group and agree with JCPDS card no. 48-1224 (a = 11.5136˚ A, b = 4.1585˚ A, and c =4.4452˚ A). No impurity phase or secondary peaks are found in any of the recorded patterns, indicating formation of phase pure SnSe in each case. The main peak corresponding 9



Figure 4: (Color online) (a) and (b) Low(2 µm) and high(400 nm) magnification FE-SEM images, (c) HRTEM image(5 nm) and (d) SAED pattern obtained for nanostructured SnSe with flake-flower (FF) morphology. to (111) plane occurs at 2θ value 30.44◦ , while the second highest peak corresponding to the (400) plane occurs at 2θ = 31.07 ◦ . As per the JCPDS file, the peak at (400) should be ∼ 50% intense as compared to the (111) main peak. However careful inspection of the profiles presented here indicate that such a ratio of intensities is observed only for the R morphology sample. We observe relatively high intensity for the (111) plane for RF and FF samples indicating it to be the preferred orientation of growth of these morphologies. The XRD profile of sample F, on the other hand, shows sudden increase in the intensity of (400) to such an extent that it surpasses the intensity of (111) plane. These relative changes in intensity indicate the preferred orientation of the crystallites in accordance with the mechanism of gowth proposed in the preceding section. Further, to explore the effect of changing morphology on the unit cell dimensions of the samples, we carried out the Rietveld refinement 29 (implemented using the FULLPROF suite 30 ) of recorded XRD patterns. Figure 5b shows a representative fit to sample R and the values of a, b and c parameters with respect to changing morphology is plotted in Figure 5c. Also, the inter-planer distances obtained from SAED analysis presented above, are in 10


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Experimental

(b)

Simulated Difference Bragg positions

Intensity (a.u)

(402)

(420)

(302)

(511)

(221) (601)

(411)

(102)

(501)

(311)

Rod Flowers(RF) (002)

(400)

(011)

(201)

(210)

(a)

Intensity (a.u)

Flake Flowers(FF) 20

30

40

50

60

2theta (deg) 11.55

(c)

30

40

50

4.450

11.53

4.445

)

Flakes(F)

20

4.455

11.54

Å

Rods(R)

Lattice constants (

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(111)

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60

11.52

4.440

a b

11.51

4.170

c 4.165

11.50

4.160 4.155

11.49 RF

2theta (deg)

R

FF

F

Samples

Figure 5: (Color online) (a) XRD patterns of all the SnSe samples. The intensity ratio between (111) and (400) planes changes as per the morphology (see text for details). Prefered orientation of (400) plane is clearly evident for flakes. (b) A representative Rietveld refinement fit to the XRD profile of rods. (c) Variation of the lattice parameters with changing morphology of the samples. accordance with these extracted values. Energy dispersive spectroscopy (EDS) was performed simultaneously while recording the FE-SEM images in order to obtain the ratio of Sn and Se in the prepared samples. The spectra were collected from different locations on the sample surface and a spatially averaged ratio of Sn:Se was obtained. All the prepared samples solely contain Sn and Se with the desired stoichiometry of 1:1. Further, the absence of oxygen in the EDS and no impurity phases or secondary peaks found in any of the recorded XRD patterns, reaffirms the formation of phase pure SnSe in each case. The performance of RF, R, FF, and F samples of SnSe in conducting electrical current at varying temperature was compared in order to understand the impact of changing morphology. For this measurement, the powder sample was compacted into 1.5 mm thick circular pellet using a hydraulic cold press. A pressure of not more than 2 tons was applied, followed by sintering of the obtained pellet at 200◦ C for 5 hr, under vacuum. The density of the final

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pellet so obtained was found to be close to ∼ 80% of theoretical density of SnSe. Automated four probe resistivity measurement was set up through computerized interface with Keithley source meter and voltmeter. The sample temperature was varied from 100 to 300 K in a closed-cycle cryostat. Figure 6a shows obtained electrical resistivity (ρ(T)) plots. A systematic fall in resistivity with increasing temperature is observed in all the samples. The room temperature resistivity (Figure 6b) of all the samples is in good agreement with the value reported in literature 24,31 for nanostructured SnSe, but significantly lower than the value (64 mΩ−m), reported for bulk polycrystalline SnSe. 22 Also, amongst the different morphologies, R, RF, and FF seem to have well-connected grains that facilitate the transport of charge carriers, while the resistivity of F in the entire temperature regime seems to be slightly higher than others. The inherently 2D nature of F causes weak inter-layer coupling resulting in much smaller contribution to charge transport in the perpendicular direction as compared to the intra-grain transport. Even after compacting into pellet forms, the underlying morphology does seem to influence the charge transport properties of nano-structured SnSe. The lowest resistivity is obtained for R sample reflecting its highly oriented single crystalline nature as confirmed by TEM image. To gain better insight of the conduction mechanism we investigate the temperature dependence of resistivity (ρ(T)) using the known models for charge transport in semiconductors. In the high temperature regime, ρ(T) is found to follow the thermally activated band conduction given by Arrhenius equation, 32 ρ = ρ0 exp ( kEBaT ), where ρ0 is the constant residual resistivity value, kB Boltzmann constant, Ea is the thermal activation energy, and T is the absolute temperature. The straight line fit to the plot of ln ρ Vs 1/T shown in the Figure 6c yields Ea values of the order of 300 meV, true to its narrow band-gap semiconducting nature. The comparative plot of the Ea values extracted for each morphology are presented in Figure 6b. The linear fit to ln ρ Vs 1/T deviates from the experimental data below 200 K. Keeping in perspective the overall magnitude of the resistivity, we apply the Efros Shklovskii

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variable range hopping (ES-VRH) model for charge transport in the low temperature regime. This process is expressed by the equation 33 ρ = ρ0 exp ( TES )0.5 , where ρ0 and TES are the T constant. The corresponding straight line fits very well to ln ρ Vs T−0.5 as demonstrated in Figure 6d. RF

Resistivity (m

R F 100

FF

320

(b)

m)

-m)

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(m

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1

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5

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ln( )

ln( )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-4

-3.0 R

R

-6

F

F

-4.5 -8 4

6

8

1000/T (K

0.072

10 -1

)

0.078

T

0.084 -1/2

(K

-1/2

0.090

)

Figure 6: (Color online) (a) Temperature dependent resistivity of all the prepared samples, (b) Variation in the room temperature resistivity values with changing morphology, (c) Arrhenius fitting in the high temperature region and (d) ES-VRH fitting in the low temperature region for prepared different morphological samples. To maintain the clarity of the figure and avoid crowding of the data, fittings for two samples is shown here. Next, the optical property of all the SnSe powder samples was analyzed using UV-VisNIR diffusion reflectance spectroscopy (Perkin-Elmer lambda 950 spectrometer). SnSe is known to show absorbance in the near-infrared region, 34 hence measurement was carried out in the wavelength range 870 - 2000 nm. The diffuse reflectance spectra obtained for all the samples are shown in Figure 7a. The samples with individual rods and flakes morphology show sharp, well-defined reflectance, as compared to its flower counterparts. The overall particle size distribution being much smaller in rods or flakes, the spatial confinement effect manifest causing narrow, sharp bandwidths and hence better relative scattering intensity. On the other hand, gradually falling reflectance in the flower forms is an indication of broad

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distribution in particle sizes and defect levels of varying degree being present in these samples. Such defect levels can affect the rate of charge carrier recombination when excited across the small optical gap. Thus, we observe that the optical behavior of SnSe does indeed get affected by its morphology. Optical band gap energy can be extracted from the diffuse reflectance data by transforming it into absorption coefficient values following the Kubelka-Munk transformation. 35 The equation, α =

A(hν−Eg )n , hν

relating the absorption coefficient, α, to the optical gap Eg , is

invoked. Here, hν is the incident photon energy, and the parameter n is assigned different values depending on the type of electronic transitions allowed in the given material. 36 For the materials with a direct band gap, n is set to a value of 1/2 and for materials with an indirect gap n equals 2. 37 Here, we extract the in-direct gap values from the present spectra by fitting a straight line to the respective ordinates and extrapolating the linear part to meet the abscissa at zero value. The corresponding fit is shown in Figure 7b, and the in-direct gap values are plotted in Figure 7c. As expected, a variation in band gap is observed with the morphology change, but strikingly low value is obtained for RF. The near negligible step feature in reflectance spectrum of RF makes it difficult to accurately determine the gap. However, the value of ∼ 0.9 eV obtained for other samples is in accordance with the values reported in literature. It may be noted that the electronic structure calculations 38 as well as experiments 39 show that the anisotropic layered crystal structure of SnSe encourages intralayer and interlayer Sn-Sn bonding, which leads to the possibility of both, direct and in-direct optical gaps to be present in this material. Accordingly, it is no surprise that many experimental reports in literature, 2,40,41 report both, direct and in-direct optical gap from the UV-Vis data of the same sample. To evaluate the potential of different morphology SnSe samples in sensing application, we studied each sample’s response towards change in humid environment. Resistive sensor device was fabricated by drop casting SnSe sample over the highly conductive indium doped tin oxide (ITO). The substrate was initially cleaned by sonication with acetone and DI water

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Reflectance (a. u.)

(a)

R F RF FF

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h

Indirect gap (eV)

(b)

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)

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Wavelength (nm)

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(F(R)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

2

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0.941

0.938 0.894

0.8

0.6

0.401 0.4

0 0.8

0.9

1.0

1.1

1.2

RF

hv (eV)

R

FF

F

Sample

Figure 7: (Color online) UV-Vis absorbance spectra recorded for all the samples of SnSe, (b) determination of in-direct band gap from the recorded spectra for two of the nominal samples, and (c) variation in the band gap values for different morphology samples.

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followed by drying in vacuum oven. No binder was used for the adhesion of SnSe film on to the substrate. All the devices were allowed to dry in a vacuum oven before carrying out further study. A schematic of the device so obtained is shown in Figure 8a. Twoprobe electrical resistance of the device was monitored by varying the relative humidity level inside a test chamber. All measurements were conducted at room temperature. A humid environment was created in a closed test chamber using saturated solutions of various salts and the relative humidity was monitored using a standard hygrometer. Figure 8b shows the dynamic response of all the four sample-devices towards the changing humidity levels. It is clear that the resistance of all four devices changes with increasing humidity. Performance of a humidity sensor is gauged from its sensitivity defined as, S=

Rini −RH RH

× 100 where Rini is the initial sensor-resistance measured at lowest humidity-

level, and RH is its value recorded upon changing the humidity in the test chamber to the subsequent level. The other equally important factor is the response & recovery time required for the absorption-desorption process. As evident from the sensitivity percentages plotted in Figure 8c, the sensor made from R sample shows best performance over the entire humidity range studied here. Incidentally, at relative humidity level ≥ 90% all the four samples show far better performance in comparison to the recent report. 26 The R sensor-device also shows a quickest response & recovery time (see Figure 8d) among the four samples. Figure 9 highlights the detailed performance of R sample as a humidity sensor. It may be noted that the sensitivity of R is almost linear between 30 to 90 % of relative humidity levels. Further, the adsorption and desorption process is highly superimposed (see Figure 9b), indicating the reversibility of both processes. As indicated in Figure 9c, the response and recovery time for this sensor is measured to be 20 and 23 seconds, respectively. These values are the lowest amongst those reported so far in the case of SnSe nanorods. 26 The assurance of stability and reproducibility of the current sensor comes from the repeatability of the measurement upon monitoring over a period of 60 days, as shown in Figure 9d. The effectiveness of a humidity sensor mainly depends on the accessibility of adsorption

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(a)

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32 (c)

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(d)

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Recovery time (sec)

0

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Sample


Figure 8: (Color online) Humidity sensing performance of different morphology SnSe samples; schematic of the device (a), change in resistance (b), sensitivity (c), and response & recovery time (d), towards changing humidity environment.

20 (a)

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18 15 )

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)

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Figure 9: (Color online) Performance of Rod-sample in terms of (a) sensitivity and resistance Vs relative humidity, (b) adsorption-desorption hysteresis, (c) response &recovery time, and (d) stability over a period of 60 days. The x-scale in (c) is a relative plot with respect to response time.

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Page 18 of 27

sites and the interaction strength between the water molecules and the adsorbate. It follows that nanostructuring of materials increases its surface to volume ratio and hence an increased access to the adsorption sites. The interaction strength, on the other hand, depends largely on the chemical nature of the adsorbate. It is well established that SnSe is a p-type material 1 with holes as the majority charge carriers, whereas water molecule posses n-type character. Upon initial exposure to humidity, the SnSe surface provides conducive bonding environment to the water molecules, establishing a charge transfer interaction. The electrons injected by water molecules shift the Fermi level of the hole-rich SnSe towards the conduction band. The data presented at Figure 8b reflects this mechanism, where the exposure to humidity decreases the sensor’s resistance to the flow of charge carriers. As the humidity level in further increased, the chemisorption is followed by physical adsorption of the next water layer. Here the water molecules attach to the chemisorbed water layer through hydrogen bonding with hydroxyl ions. With increase in the humidity level, the physisorbed water layer starts to build generating a electrolytic proton conduction. This is a very well known process called the Grotthuss mechanism, 42 described on the basis of dissociation of water into H3 O+ and OH− by electrostatic field. Accordingly, the conductivity increases, further plunging down the sensor-resistance. This process is evident from the Figure 8c, where impressive gain in humidity sensing is observed beyond 90% of relative humidity level. Table 2: Adsorption capacity (kf ), adsorption strength(α) and the goodness of fit (R2 ) extracted upon fitting the Freundlich isothermal adsorption model to the measured humidity sensing data for different morphological SnSe sensors. Sample kf RF 0.16 ± 0.21 R 4.02 ± 0.07 FF 1.02 ± 0.18 F 0.27 ± 0.15

0.72 1.44 1.02 0.78

α ± 0.11 ± 0.04 ± 0.09 ± 0.08

R2 0.96 0.98 0.95 0.97

The relation between concentration of water molecules adsorbed on SnSe sensor surface and the relative humidity to which the sensor is exposed, can be stated using the Freundlich isothermal adsorption model. 43 The empirical relation describing the essence of this model 18


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2.0

RF R

H

R )

(b)

(a)

1.5

log (

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FF

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Linear fit to the model

region-1

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1.8

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1.0

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log(Percent Relativie Humidity)

Figure 10: (Color online) Freundlich isothermal adsorption model fitting for different morphological SnSe sensors. (a) log – log plot of relative resistance change Vs. relative humidity, (b) Linear fit to the data for R sample. can be stated as, q = kf C α , where q represents the maximum adsorption capacity and C is the equilibrium concentration of the solution. The Freundlich constant (kf ) represents the adsorption capacity and and α stands for adsorption strength. Adapting this model to the experimental data, the relative change in resistance of the sensor can be taken as the measure of adsorption capacity, and the relative humidity level maintained in the test chamber corresponds to the equilibrium concentration. Thus a plot as presented in Figure 10a is obtained. Clearly, the response of the system can be divided into low and high humidity regions. Straight line fit to the data in these two regions yeilds the parameters, kf and α from the intercept and slope, respectively. In region 1, all the four samples seem to have nearly same intercept while the slope is greater than 1 indicating chemisorption of water molecules on the sensor surface; the highest adsoprtion strength being displayed by the R sample. The fit to region 2 and the parameters extracted, are shown in Figure 10b and Table 2, respectively. As the humidity levels go up, the relative adsorption strength of R and FF though become comparable, highest adsorption capacity is obtained for R (4.02) which supports its superior performance as a sensor. On the basis of the Freundlich model fit to the measured data, the variance in sensing performance of the four SnSe sensors can be quantified in terms of surface functionality 19



on RF, R, FF, and F towards adsorption of water molecules. Due to its highly ordered crystal structure and intrinsically low electrical resistance, sensors made from R morphology finds easy establishment of charge transfer with water molecules with much lower activation energy. On the other hand, weak interlayer coupling and higher intrinsic electrical resistance may be the reason for poor performance of flakes. The flower morphology can provide better pores for capillary action in the physical absorption phase. But the multiple defects at band structure level (as seen from its optical properties) bring about serious drawbacks. Such defect materials imply a heterogeneous adsorbent surface having varying degrees of affinity for the adsorbate binding sites. The desorption of the water molecules also becomes difficult implying a longer recovery time and hence limiting its effectiveness as a sensor. Having found good response as a resistive sensor, it becomes pertinent to know the thermal stability and degradation against temperature rise in SnSe samples. An investigation to that effect was hence carried out by conducting the thermo-gravimetric analysis (TGA) of all four samples. Figure 11 shows the obtained TGA pattern for each sample, measured as a function of temperature from RT to 1000◦ C, with the heating rate of 10◦ C/min in N2 atmosphere. All the samples show a similar weight loss pattern; a small gain around 575◦ C indicates adsorption of atmospheric N2 . Beyond 580◦ C, a downward trend in weight percentage begins and noticeable weight loss is seen in the range 600-760◦ C. The associated weight loss of 32% corresponds to weight percentage of Se atoms and indicates the decomposition of SnSe to Sn + Se. Thus in the safe limit, the present samples of SnSe are stable upto ∼ 550 ◦ C.

4

Conclusion

We prepared four samples of SnSe nanostructures with an aim to evaluate the impact of changing morphology on its structural, electrical, optical and sensing properties. An exact recipe for obtaining different morphology structures like rods, flowers, flakes, flake-flowers,

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110 0.00 0.00

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-0.12 200

400

600

800

Temperature ( C)

Figure 11: (Color online) TGA-DTA curves recorded for different morphology samples. from the same precursor solution in a hydrothermal method of synthesis is presented. Morphology of samples and growth direction was confirmed by FE-SEM and HR-TEM images. Thorough analysis of the crystal structure using least squares fit to the XRD profiles and SAED patterns provide the lattice parameters. Electrical transport measurement carried out at different temperatures highlight the increasingly 2D nature of the nano-flakes and the role played by hopping conduction mechanisms at low temperature. Optical gap energy seem to change with the morphology, but the variation in optical absorbance and sharp transitions appear better in individual rods and flakes. Resistive sensors for humidity sensing application show quite good sensitivity, good repeatability, low hysteresis and good reproducibility at room temperature. This work reveals the morphology driven application of a given material and serves to assist the miniaturising technology for multifunction materials.

Acknowledgement This work is based upon the project supported by Department of Science and Technology (DST), New Delhi (Grant No:SR/S2/CMP-0109/2012). The authors thank Sophisticated

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Instrumentation Centre (SIC) at IIT Indore for providing the FE-SEM and TGA characterization facilities. Central Instrumentation Facility (CIF) at IIT Guwahati is hereby acknowledged for extending the TEM facility. G. S. thanks Ministry of Human Resource Development (MHRD), Government of India, for teaching assistantship and P.M.S. thanks DST-SERB for Ramanujan fellowship (SR/S2/RJN-121/2012).

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Graphical TOC Entry

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Impact of Different Morphological Structures on Physical Properties of

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