Porous Silicon Nanostructures for

Article 2

Wafer-scale synthesized MoS/Porous Silicon nanostructures for efficient and selective ethanol sensing at room temperature Priyanka Dwivedi, Samaresh Das, and Saakshi Dhanekar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 29 May 2017 Downloaded from http://pubs.acs.org on May 31, 2017

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ACS Applied Materials & Interfaces

Wafer-scale synthesized MoS2/Porous Silicon nanostructures for efficient and selective ethanol sensing at room temperature Priyanka Dwivedi, Samaresh Das*and Saakshi Dhanekar Centre for Applied Research in Electronics, Indian Institute of Technology (IIT), Hauz Khas, New Delhi – 110016, India *E-mail: [email protected]

KEYWORDS: Molybdenum disulfide (MoS2), layered materials, heterojunctions, VOC sensor, porous silicon Acronyms: c-Si: Crystalline silicon, VOC: Volatile organic compound, IPA: Isopropyl alcohol, IDE: interdigitated electrode, PSi: Porous silicon, NFs: Nanoflakes, MoS2: Molybdenum Disulfide, RCA: Radio Corporation of America ABSTRACT This paper presents the performance of a highly selective ethanol sensor based on MoS2functionalized porous silicon (PSi). The uniqueness of the sensor includes its method of fabrication, wafer scalability, affinity for ethanol and high sensitivity. MoS2 nanoflakes (NFs) were synthesized by sulfurization of oxidized RF-sputtered Mo thin films. The MoS2 NFs synthesis technique is superior in comparison to other methods as it is chip scalable and low in cost. Interdigitated electrodes (IDEs) were used to record resistive measurements from MoS2/PSi sensors in the presence of volatile organic compound and water vapors at room temperature. With the effect of MoS2 on PSi, an enhancement in sensitivity and a selective response for ethanol were observed, with a minimum detection limit of 1 ppm. The ethanol sensitivity was found to increase by 5 times in comparison to the single-layer counterpart levels. This impressive response is explained on the basis of an analytical resistive model, the band gap of MoS2/PSi/Si, interface formed between MoS2 and PSi, and the chemical interaction of the vapor molecules and

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the surface. This 2D composite material with PSi paves the way for efficient, highly responsive and stable sensors. 1. INTRODUCTION The capacity of a gas sensor is gauged by its ability to sense a gas selectively. This depends on various factors such as the sensing material, the gas, the sensing mechanism and other environmental factors1–3. One of the ways to improve sensing performance is to use nanostructures rather than bulk materials, which grant high surface area-to-volume ratios, good adsorption properties, more reactive sites for redox reactions and high surface reactivity4,5,6. Ethanol sensing has become a common practice for assessing the amount of alcohol present in human breath. Various groups are working in this field in an attempt to fabricate low-cost and field-deployable sensors. The sensitive materials that are being used in ethanol sensors include metal oxides7–9, porous structures10 and 2D materials.11–14 Recently, 2D transition metal dichalcogenides (TMDCs) have shown improved and selective gas sensing performance due to their varying affinity for different analytes.15,16 MoS2 is a 2D material and one of the potential candidates for gas sensing applications. Single- and multilayer MoS2 devices based on IDEs and FETs have shown high sensing performance of NH3, NO, NO2, VOCs and water vapors.15,17,18 Multi-layered MoS2 structures are preferred over their single-layer counterparts due to their scalable fabrication process and stable sensing response characteristics. Another group of materials that have attracted much interest due to their impressive properties for gas sensor applications are porous silicon (PSi)-based compounds. The most attractive feature of PSi is its compatibility with the modern IC industry, as devices based on PSi can be easily integrated with other components on the same board. The key role played by PSi in this work is to lower the sensor operating temperature.19–21


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Another class of sensitive materials are metal oxides, which are known to operate at high temperatures22,23. Arafat et al. presented TiO2 nanoparticle-based ethanol sensor operation at 600 °C. Another study by Park et al.24 reported ethanol sensing operable at 150 °C. Few reports in the literature are available in which the sensor response from 2D materials is observed at elevated temperatures25. Porous structures, however, are meant to be extremely sensitive, and they exhibit stability issues; therefore, they need to be in situ treated

26

or post-fabrication treated for

improved stability10. To circumvent these operating temperature and stability issues, heterojunctions are used, and it has been reported that these interfaces are more sensitive to gases through a mechanism known as “synergistic reaction”27. A depletion width is formed due to the Fermi level mismatch in a heterojunction. This depletion width is modulated and carrier movement occurs upon exposure of this interface to analytes. Other issues that hinder fast prototyping are batch processing and cost of fabrication. Both are dependent on the choice of fabrication process. Some of the techniques for wafer scaling are dry processes, including sputtering and CVD5. The other techniques used for sensor fabrication involve wet techniques, which are not found to be successful in the formation of layers with uniform thickness and purity.28,29 In this paper, we present wafer-scale synthesis of MoS2/PSi for use as a selective ethanol sensor at room temperature. The MoS2/PSi heterojunction was explored as a sensing platform to improve selectivity, sensitivity and sensor operating temperature. Samples with different thicknesses of Mo were oxidized and later were subjected to sulfurization to form MoS2. Sensing tests were performed at room temperature in different VOCs and moisture levels. A selective response to ethanol was shown by the MoS2-functionalized PSi, and samples with comparatively thinner layer of MoS2 demonstrated higher sensitivity. To strengthen our hypothesis, a



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comprehensive study was done in which the response of MoS2/PSi was compared with those of individual MoS2 and PSi films. It was determined through long-term studies that the sensor was stable for a period of 60 days. The role of the 2D material on the porous substrate and their interface has been highlighted in sensing applications. 2. EXPERIMENTAL 2.1. Sensor device fabrication using MoS2-functionalized Porous Silicon (PSi) The devices were fabricated using p-type, orientation, 1-10 Ω-cm resistivity silicon wafers. The wafers were thoroughly cleaned using the RCA procedure to remove any dirt or contamination from the silicon surface. The fabrication process flow is shown in Figure 1 and briefly explained as follows. The silicon substrate was electrochemically etched using an anodization technique with a single-cell configuration in an electrolytic solution consisting of HF:ethanol (1:1) at 50 mA.cm-2 for 15 minutes.30 This formed PSi that consisted of several pores embedded in silicon nanocrystallites. Different thicknesses (10 and 20 nm) of Mo were deposited on silicon and PSi substrates using an RF sputtering technique at an RF power of 150 W and a pressure of 10 mtorr. These were oxidized to form MoO3 in 100% oxygen atmosphere at 500 °C for 1 hour.5 The samples were further subjected to sulfurization in a horizontal tube furnace containing a small quartz boat with sulfur powder (1.2 g) next to the MoO3 samples. MoS2 NFs were formed on PSi samples by annealing at 600 °C for 20 minutes with a N2 flow rate of 1 liter/minute. During this process, suboxide compounds diffuse to the substrate and further react with sulfur vapors to grow MoS2 NFs. IDE structures with a gap and linewidth of 300 µm each were formed using a metal mask by depositing Cr/Au (20 nm/100 nm) by


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RF sputtering and were classified in accordance with their preparation conditions and surface functionalization, as listed in Table 1. 2.2 Characterization and sensing measurement of as-synthesized MoS2-functionalized Porous Silicon (PSi) The top surface and cross-sectional morphology analysis of the MoS2 NFs and PSi were examined using SEM (Zeiss EVO 50) and FE-SEM (FEI Quanta 200 F SEM). The crystal structures of the samples were analyzed by XRD (Phillips X’Pert, PRO-PW 3040 diffractometer) with a glancing angle of 0.5°. Structural analysis was performed using Raman spectroscopy (LAbRAMHR Evolution RAMAN Spectrometer (Horiba)) with an Ar laser excitation wavelength of 514 nm. Electrical detection was performed using the sensing setup interfaced with a Keithley 6514 electrometer. Data were acquired using an acquisition system using LABVIEW software for real-time sensing measurements. X-ray photoelectron spectroscopy (XPS) data were obtained from a PHI Quantes scanning dual X-ray photoelectron microprobe, ULVAC-PHI Inc. under a basic pressure of 10-8 torr. 3. RESULTS AND DISCUSSION 3.1. Structural Analysis SEM is used to investigate the morphology of the nanostructured samples. The SEM micrographs and formation of MoS2 NFs in samples S1 (MoS2/c-Si), S2 (MoS2/PSi) and S3 (MoS2/PSi) are shown in Figure 2. The slight changes in morphology of the samples in Figures 2(b) and (c) from that in 2(a) can be attributed to the change in substrate. Figure 2(a) shows a crystalline silicon substrate, whereas Figures 2(b) and (c) show a PSi substrate. An SEM image of PSi is depicted in the inset of Figure 2(b). This image shows a pore size of approximately 4.3 nm. The NFs formed on a porous substrate seem to be more flattened out and more uniform in size and shape than those



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formed on c-Si. Additionally, the effect of Mo thickness is reflected in Figures 2(b) and (c). The NFs in samples with lesser Mo thickness are less dense than their thicker counterparts. Figure 2(d) depicts a cross-sectional view of sample S2. It shows a pore depth of approximately 2 microns with MoS2 NFs on top. EDX analysis (Figure 2(e)) confirmed the existence of elements such as Mo, S, O, and Si. The structural details of the MoS2 crystal were thoroughly studied using XRD patterns. The peaks for the S1, S2 and S3 samples were scanned in from 10–70° as shown in Figure 3. The obtained peak positions corresponding to 2theta are comparable and consistent with the standard JCPDS card No.371492. For samples S1 and S2, peak positions were observed at 14.4°, 25.5°, 39°, 52.6° and 58.5°, which were assigned to the (002), (004), (103), (105) and (110) planes, respectively.13,31 For the thicker Mo in sample S3, peaks were observed at 14°, 30°, 37°, 52.6° and 58.6°. The diffraction peak at 14° is approximately the same for both S1 and S2, whereas the peak at 25.5° is more intense for S3 in comparison to S1 and S2. A peak shift was observed in sample S3 from 39° to 37° for the (103) plane, which is due to lesser thickness of Mo on the PSi sample. Many structural properties of a material can be obtained using Raman spectroscopy, namely, structural features, orientation of the facets in the crystal, transition of the material from one phase to another, bonding details, thermal conductivity, stress and compositional details.32 Figure 4 depicts the detailed Raman spectroscopy studies for all samples. Figure 4(a) shows the Raman spectra of the S1, S2 and S3 samples displaying the in-plane and out-of-plane E2g and A1g vibrational modes, respectively. The effect of thickness on the peak positions was clearly observed by the shifting of the E2g and A1g modes. The peak positional difference (delta) between the two E2g and A1g bands was


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calculated as 23.7 cm-1, 23.5 cm-1 and 26 cm-1 for samples S1, S2 and S3, respectively. This difference (delta) represents the number of MoS2 layers.33 Therefore, after analysis, samples S1 and S2 show multilayer (~trilayer) MoS2, and the Raman spectrum of sample S3 with a Mo thickness of 20 nm refers to bulk MoS2. This demonstrates that as Mo thickness increases, the positions of modes shift, and the material exhibits bulk nature. Figure 4(b) displays optical images of samples S1 (MoS2/c-Si), S2 (MoS2/PSi) and S3 (MoS2/PSi) to visually distinguish the samples on the basis of their color change. Sample S3 with higher Mo thickness appears clearly different from S1 and S2. A large-scale structural analysis using Raman spectroscopy at room temperature was performed. Raman mapping confirms the uniform and large-scale growth of MoS2 NFs. Figure 4(c)-(d) depicts the Raman mapping areas corresponding to the E2g and A1g bands, respectively. The scanned Raman area was 10×10 µm2 with 0.8 µm steps. This confirms the uniform and large-scale fabrication of NFs. X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA), is a technique to investigate the chemical composition and binding energy of surfaces. Figure 5 depicts the wide XPS spectrum of the synthesized MoS2 sample S2, which exhibits only signals arising from the elements Mo and S. The Mo 3d XPS spectrum of MoS2 shows two strong peaks at 229.3 and 232.5 eV, attributed to Mo 3d5/2 and Mo 3d3/2, respectively.34 Peaks at 162.2 and 163.2 eV corresponding to the S 2p1/2 and S 2p3/2 orbitals of divalent sulfide ions (S2−), respectively, are observed. Since the sample was MoS2-functionalized PSi, the two intense peaks at 103.6 and 153.75 eV correspond to Si 2p and Si 2s, respectively.31 The Gaussian peak at 103.6 eV pertains to the Si4+ oxidation state.35



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4. SENSING STUDIES 4.1 Dynamic Response The sensor responses were recorded in real time using the electrometer and the LABVIEW data acquisition software. The sensor setup is shown elsewhere.21 Briefly, the sensor was placed in a test chamber that had inlets for analyte vapors and carrier gas (nitrogen). The analyte was placed in a separate container that was kept at a controlled temperature to maintain the vapor pressure. The concentration of vapor inside the sensing chamber was established using the relation of the analyte vapor pressure and ppm. The change in electrical resistance of the sensor was observed after it was exposed to analytes. The sensors were tested in the presence of vapors such as methanol, ethanol, acetone, IPA, benzene, toluene and moisture. The sensor response (S) for analyte vapors was calculated using the following relation,

% =

× 100,……….(i)

where is the resistance in the presence of analyte vapors and is the resistance of the sample in air. Figure 6 displays the sensing characteristics of sensors S1, S2 and S3. Figure 6(a) shows the dynamic response of sensors S1, S2 and S3 to air and different concentrations of ethanol vapors at room temperature. To assess the sensing performance of the MoS2-functionalized PSi sample, sample S1 (MoS2/c-Si) was also tested in the same conditions and it was found to be least responsive to ethanol in comparison to samples S2 and S3. This can be observed from Figures 6(a) and (b). Figure 6(b) depicts that among all samples, S2 (MoS2/PSi) with 10 nm of Mo on PSi was found to be most sensitive to ethanol. Its response was found to be almost linear and increased by 5 times in comparison to that of S1. This shows the significant role played by the substrate in the


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sensing behavior. The sensor response was approximately 2% at 1 ppm concentration, which is close to real world applications. Thus, the sensor can be used to detect wide concentrations of ethanol. The limit of detection (LOD) was calculated using this graph. Theoretically, it is defined as three times the standard deviation and was calculated to be 0.7 ppm for sample S2. The sensor response obtained from sample S2 in the presence of 20 ppm of each test analyte is shown in Figure 6(c). This sensor was tested for almost 90 days, and little difference in the response was observed (Figure 6(d)). This affirms that the sensor exhibits repeatability and long-term stability. The sensitivity was calculated as slope of the response unit/ppm. Figure 6(e) depicts the sensitivities of samples S1 and S2 and the sample with PSi alone. It shows that sample S2 was selective and the most sensitive to ethanol. A possible explanation for this sensing behavior is given in the following section. 4.2 Sensing Mechanism The sensing response of a gas sensor depends on many factors, such as the sensing material morphology, carrier concentration, temperature, Debye length, adsorptiondesorption coefficients, orbital energy and the reaction rate of gas molecules on the surface and catalytic properties of the material.36 The sensor discussed in this work can be represented as shown in Figure 7(a), with its equivalent electrical circuit in Figure 7(b). In the electrical resistance detection mechanism, the effective resistance of the device plays a dominant role. The conduction mechanism of (MoS2/PSi) sensor device can be explained by considering several resistances present in various materials and at different interfaces. These resistances are those of the Au/Cr and MoS2 interface (RAu/Cr/MoS2), MoS2 (RMoS2), the MoS2 and PSi interface (RMoS2/PSi), PSi (RPSi), the PSi and c-Si interface (RPSi/c-Si) and c-Si (Rc-Si). Reffective is the equivalent resistance of the four



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resistances RMoS2/PSi, RPSi, RPSi/c-Si and Rc-Si. During the sensing process, the change in resistance due to both adsorption and desorption will be mainly affected by two resistances, RMoS2 and Reffective shown in its simplified form. The changes in the surface properties of a sensor are due to physisorption or chemisorption, or sometimes both, of the test analyte vapors. Since MoS2 is formed in the form of NFs, it provides a large surface area-to-volume ratio for the adsorption of vapors. However, while the overall interaction mechanism of vapors with MoS2 is still under debate, we could conclude from Figure 6(a) that chemisorption of ethanol occurs, as there is a slight shift in the baseline. Its complete desorption from MoS2 does not occur, and thus, the baseline is not recovered.37,38 The other contributors to the sensor response could be sulfur vacancies, defect states, unstable bonds at the surface, the nature of the analyte and various other physical and chemical properties of the surface.39 Figures 8 (i) and (ii) show the energy band diagrams of S1 and S2, respectively, where PSi/c-Si forms a p-p junction and MoS2/PSi forms a p-n junction. Since MoS2 is an n-type semiconductor and PSi is p-type, electrons and holes are the majority carriers on the surfaces of MoS2 and Psi, respectively. The selectivity of a sensor depends on the interaction of the analyte vapor and the sensing surface. This relates to the chemical properties of both the surface and target. The alcohols tested have single bonds, whereas acetone, benzene and toluene have double bonds. Since toluene and benzene are non-polar, the interaction of these compounds with the sensor surface is minimal. The response to toluene is greater than that to benzene, however. This could be due to the presence of methyl groups causing a dipole-induced scattering. Bond dissociation energy is required to break a bond to create a reaction with


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the surface, and since acetone has a double bond therefore, acetone require has a double bond it require more energy and thus acetone also gives a lower response. The response from methanol vapors is found to be close to, but slightly less than, that from ethanol. This could be attributed to its lower mass and volatile nature. As the hydrocarbon chain length increases, the mass increases, and the difficulty in breaking the –OH bond increases. This could be the reason for the surface reacting to ethanol and not to IPA. Of the three hydrocarbon chain molecules, methanol, ethanol and IPA, ethanol gives the maximum response due to the following reaction: MoS2 + C2H5OH (vap) = MoS2- − C2H5OH+ (ads)

………….(ii)

MoS2- − C2H5OH+(ads) = MoS2 + C2H5OH+ (ads) + e-CB …….(iii) where the ads suffix means “adsorbed” and e-CB is the electron in the conduction band. These electrons recombine with holes and modulate the depletion width. This can be explained with the help of the band diagrams of MoS2/c-Si and MoS2/PSi shown in Figure 8. The band gap of PSi is greater than that of c-Si due to quantum confinement. Thus, in the case of MoS2/PSi, the barrier height for electrons is lower in comparison to that of MoS2/c-Si; therefore, the probability of recombination at the interface is higher. Due to this, the depletion width increases, resulting in a change in resistance. Since more recombination occurs at the MoS2/PSi interface, it is therefore more likely to be sensitive than MoS2/c-Si. The same is observed from the resistance model, where the change in resistance is greater in MoS2/PSi. This was corroborated from Figure 7(b), where Reffective is more dominant than RMoS2. This might also be the reason for the heterostructure displaying p-type behavior and displaying an increase in resistance upon exposure to



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reducing agents. To compare our sensor characteristics with those of other sensors reported in the literature, a comprehensive study has been performed and is listed in Table 2. 5. CONCLUSION The process developed for MoS2/PSi sensor is scalable, reproducible, cost effective and CMOS compatible. Electrical sensing in the presence of VOCs and moisture was performed by forming IDEs on the surfaces at room temperature. MoS2/PSi samples were found to be stable, most sensitive to ethanol, with a low limit of detection at 1 ppm. Samples with a MoS2 thin film or PSi alone were not found to be very sensitive. The sensing mechanism was explained on the basis of the p-n heterojunction formed by PSi and MoS2. The effects of this junction are why the sensitivity was found to be enhanced and observable at low ppm. This study provides insights into the role of PSi, 2D material nanostructures and interface of MoS2 and PSi for the selective sensing of ethanol with the added perspective of the development of a wafer-scalable process used for the sensor development.

List of Figures:


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Figure 1 Fabrication process flow of the sensor devices.



Figure 2 SEM micrographs. (a) Sample S1: c-Si/MoS2. (b) Sample S2:MoS2/PSi. (c)

2

S2 PSi S2 MoS MoS22/ /PS (110)

(103)

(105)

(110)

(103)

(105)

S1 MoS /P silicon 2 MoS /c-Si

20

S2 MoS2 //PS PSi S3

40

(110)

(105)

(103)

(002)

(004)

(004)

(002)

(004)

(002)

Sample S3: MoS2/PSi. (d) Cross-sectional SEM of S2:MoS2/PSi (e) EDX spectrum of S2.

Intensity (a.u)

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60

Position (2θθ) Figure 3 XRD patterns of MoS2-functionalized PSi samples.


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Figure 4 Samples S1, S2 and S3 (a) Raman spectra. (b) Optical images of samples S1, S2 and S3. (c) Raman mapping of sample S2 for E2g intensity and (d) A1g intensity.



S 2P3/2

Mo 3d3/2

Intensity (a.u)

Mo 3d5/2 S 2P1/2

S 2s

226

228

230

232

234

160

162

164

Si-2p

Si-2s

Intensity (a.u) 145

150

155

160

165

95

Binding Energy (eV)

100

105

110

Binding Energy (eV)

Figure 5 X-ray photoelectron spectroscopy (XPS) analysis of MoS2-functionalized PSi sample S2.

3

Analyte on

1 ppm

2

15

Response %

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40 ppm

(a)

Analyte off

1 0 0

10

10

20

30

20 ppm

S2

10 ppm 5 ppm

5

S3 S1

0

In presence of air 0

300

600

900

1200

Time (sec.)


1500

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20 S1: MoS2/ p silicon S2: MoS2/ PSi S3: MoS2/ PSi

15

Response %

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(b)

10

5

0 0

5

10

15

20

25

30

ppm


35

40


(d)

10

Sensor Response %

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@ 20 ppm (ethanol vapors)

(e)

8

6

4

50

100

150

200

Temp (°C)

Figure 6 Sensing study of samples S1, S2 and S3. (a) Dynamic response in the presence of different ethanol vapor concentrations, (b) Response% versus ethanol concentration (ppm) (c) Stability studies of sample S2: MoS2/PSi sensor at 10 ppm ethanol vapor tested for a duration of


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two months. (d) Sensitivity graph of all samples exposed to different analytes. (e) Temperature study of MoS2/PSi sample.

Cr/Au MoS2 PSi c-Si

(a)

Figure 7 (a) Schematic diagram of a MoS2/PSi sensor device. (b) Equivalent electrical resistance model of a MoS2/PSi sensor device and its simplified version (Rj– junction resistance and Cj – junction capacitance).



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(i) Ec Ec = 0.07 eV Eg >1.1ev

Eg (1.6ev)

Eg (1.1ev)

Ef Ev

PSi EV = 0.09 eV

Ec (ii) EC = 0.16 eV

Eg (1.1ev)

Ef Ev

Eg (1.6ev) EV = 0.34 eV

(p silicon)

(MoS2)

Figure 8 (i) Band diagram of MoS2/PSi/c-Si. (ii) Band diagram of MoS2/c-Si.

Table 1. Sample preparation parameters Sample

Substrate

Name

Mo

Deposition

Oxidation

Sulfurization

thickness

time (mins.)

Temp.

Temp.

S1

c-Si

10 nm

2

500 °C

600 °C

S2

PSi

10 nm

2

500 °C

600 °C

S3

PSi

20 nm

4

500 °C

600 °C


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Table 2. A comprehensive survey on already reported sensors S. No.

Nanostructured Materials

Fabrication Method

Morphology

Sensing Technique

Target Analyte

Ref.

1.

Au-Modified MoS2

Chemical route

Nanosheets

Fluorescence

DNA

40

2.

MoS2

Chemical route

Nanosheets

Fluorescence

41

3. 4.

MoS2 MoS2

Chemical route _

Nanoflakes Flakes

5.

SnO2 /MoS2

Low-temperature Hydrothermal

Nanoparticle/ Nanosheets

Fluorescence FET channel conductance (∆ID/ID) Electrical detection (R)

DNA methyltrans ferase Glucose Ethanol

25

6.

MoS2-Decorated TiO2

Hydrothermal

Nanotube

Electrical detection (R)

7.

MoS2

Chemical route

Nanoparticle


8.

Ag-TiO2

Sol gel

Nanoparticles Electrical detection (R)

9.

Cr2O3−ZnO

Thermal evaporationsolvothermal

Nanoparticles Electrical /nanorods detection (R)

10.

α-Fe2O3

Hydrothermal

Nanospheres

11.

TiO2

-


12.

Sb doped SnO2 nanowire

CVD

Nanowires

13.

TiO2

Chemical route


14.

TiO2/PSi

Reactive Sputtering

Porous morphology

Ethanol (50ppm) @280 °C Ethanol (50ppm) @150 °C Ethanol (3 ppm) @200 °C Ethanol (3 ppm) @200 °C Ethanol (200 ppm) @Room Temperatur e (RT) Ethanol (10 ppm) @240 °C Ethanol (20 ppm) @600 °C Ethanol 40ppm @ RT Ethanol @150 °C 5ppm Ethanol 5ppm





42 14

12

28

43

44

45

46

47

48

21


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

15.

Bi2S3 nanowires

Hydrothermal

Nanowires


16.

MoS2/PSi

Sputtering/Anodi zation

Nanoflakes/P orous Si


Page 22 of 27

@ RT Ethanol 20ppm @ RT Ethanol 1ppm @ RT

49

This work

Corresponding Author *Tel: +91-11-26596036; Email: [email protected] (Dr. Samaresh Das) Author Contributions The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS The authors gratefully acknowledge the Central Research Facility (CRF) and the Nanoscale Research Facility (NRF), Indian Institute of Technology (IIT) Delhi for providing characterization facilitates. The first author would like to acknowledge MHRD, Government of India for providing a financial assistantship for research. The third author would like to thank the Department of Science and Technology (DST), Government of India for its financial support through its grant no. INSPIRE Faculty award IFA-12 ENG13. The authors thank Bio Nano Electronics Research Center, Toyo University, Japan for XPS analysis. REFERENCES (1)

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TOC 254x190mm (300 x 300 DPI)


Porous Silicon Nanostructures for

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