TaS3 Nanofibers: Layered Trichalcogenide for High-Performance

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TaS3 Nanofibers: Layered Trichalcogenide for High Performance Electronic and Sensing Devices Carmen C. Mayorga-Martínez, Zdenek Sofer, Jan Luxa, Stepan Huber, David Sedmidubsky, Petr Brázda, Lukas Palatinus, Martin Mikulics, Petr Lazar, Rostislav Medlín, and Martin Pumera ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b06853 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 12, 2017

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TaS3 Nanofibers: Layered Trichalcogenide for High Performance Electronic and Sensing Devices Carmen C. Mayorga-Martinez,1§ Zdeněk Sofer,*2§ Jan Luxa,2 Štěpán Huber,2 David Sedmidubský,2 Petr Brázda,3 Lukáš Palatinus,3 Martin Mikulics,4 Petr Lazar,5 Rostislav Medlín6 and Martin Pumera*1,2 [email protected] [email protected]

1

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical

Sciences Nanyang Technological University, Nanyang Link 21, Singapore 637371, Singapore 2

Department of Inorganic Chemistry, University of Chemistry and Technology Prague,

Technická 5, 166 28 Prague 6, Czech Republic 3

Institute of Physics of the CAS, v.v.i., Na Slovance 2, 182 00 Prague 8, Czech Republic

4

Peter Grünberg Institute (PGI-9), Forschungszentrum Jülich, D-52425 Jülich, Germany and

Jülich-Aachen Research Alliance, JARA, Fundamentals of Future Information Technology, D-52425 Jülich, Germany 5

Department of Physical Chemistry and Regional Centre of Advanced Technologies and

Materials, Palacký University Olomouc, tř. 17. Listopadu 1192/12, 771 46 Olomouc, Czech Republic 6

New Technologies - Research Centre, University of West Bohemia, Univerzitní 8, 306 14

Plzeň, Czech Republic 1 ACS Paragon Plus Environment

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§ These authors contributed equally

KEYWORDS: layered trisulfide; gas sensor; photodetector; field-effect transistor; air pollutants.

ABSTRACT The layered materials like transition metal dichalcogenides exhibit broad spectra of outstanding properties with huge application potential, while another group of related materials, layered transition metal trichalcogenides remains unexplored. Here we show a broad application potential of this interesting structural type of layered tantalum trisulfide prepared in a form of nanofibers. This material shows tailorable attractive electronic properties dependent on the tensile strain applied to it. Structure of this so-called orthorhombic phase of TaS3 grown in a form of long nanofibers has been solved and refined. Taking advantage of these capabilities, we demonstrate highly specific impedimetric NO gas sensor based on TaS3 nanofibers as well as construction of photodetectors with excellent responsivity and field effect transistors. Various flexible substrates were used for the construction of NO gas sensor. Such device exhibits a low limit of detection of 0.48 ppb, well under the allowed value set by environmental agencies for NOx (50 ppb). Moreover, this NO gas sensor also proved excellent selectivity in the presence of common interferences formed during fuel combustion. TaS3 nanofibers produced in large scale exhibited excellent broad application potential for various types of devices covering nanoelectronic, optoelectronic and gas sensing applications.

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The emergence of two dimensional materials like graphene, transitional metal dichalcogenides (TMDs) and black phosphorus have propelled the rapid advancement of electronic, optical and chemical gas sensors devices.1-10 Apart from TMDs, transition metal trichalcogenides (TMTs) are a class of nanomaterials that have recently attracted attention of researchers due to their electronic properties that range from insulator to metallic conductivity11,12 for energy applications, optical and electronics devices.13 In particular, TaS3 is an interesting TMT material which has a highly conductive and lamellar crystal structure, whose unit cell consists of S−Ta−S sandwich layers growing in a form of long nanofibers.13 It also shows high capability for energy storage.13 Earlier studies have revealed interesting physical phenomena in TaS3 such as formation and travelling of charge density waves (CDW), accompanied by metallic to semiconducting transition.14-16 In this way, a fundamental study focused on detailed structure characteristics and further applications of this outstanding material in electronic, optical and sensing areas, has not been done yet. Among these advanced applications, gas sensors based on two dimensional materials like transitional metal dichalcogenides (TMDs) and black phosphorus (BP), possess attractive benefits such as high sensitivity and selectivity.1,

3, 5

Gas sensing is indeed essential for many aspects of

modern society ranging from automotive industry settings to smart cities. Of particular importance is the sensing of nitrogen oxides (NOx), mainly NO and NO2, which are toxic gases associated with combustion sources and causing environmental and human health problems. Moreover, NO is oxidized in air to form NO2 which in turn reacts with water to produce nitrous acid (HNO2) and nitric acid (HNO3).17 Nitric and nitrous acid contribute to the formation of acidic rains which have harmful effects on plants, aquatic animals and infrastructure. Due to all these reasons, controls have been placed by environmental agencies on the emission of these pollutants to ensure that they do not exceed 50 ppb.6, 17 From this point of view, there are demands for highly sensitive and low-cost gas sensors for NOx


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detection and the transitional metal chalcogenide materials are good candidates for achieving this aim.18 Here, we performed a systematic fundamental study on the structure of so-called orthorhombic TaS3 nanofibers. The structure of TaS3 was solved and refined using data from precession electron diffraction tomography (EDT). These results also show a monoclinic form as the second already known polymorph of TaS3 (P21/m), while the discovered form was successfully indexed within the C2/m space group. As both polymorphs are monoclinic, the old notation using the terms “monoclinic” (P21/m) and “orthorhombic” are misleading. We thus use the notation P-TaS3 for P21/m and C-TaS3 for C2/m throughout the text when it is necessary to distinguish these phases. The obtained model was used for electronic structure and phonon spectra calculations and the results were correlated with the recorded Raman spectra. Moreover, we used TaS3 nanofibers exceeding hundreds of microns in length and obtained in large scale for construction of various devices like field-effect transistor (FET), optoelectronic detectors and highly selective impedimetric gas sensors (see Figure 1). Moreover, the NO gas sensor based on TaS3 nanofibers reported here shows highly sensitivity and selectivity. Taking advantage of the great length of the nanofibers, we implemented an easy-to-manufacture gas sensor in a resistor mode, where the fibers were connected by silver ink onto different substrates such as polyester, parafilm and paper (see Figure S1).


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Figure 1. Schematic representation of various TaS3 nanofibers based devices including fieldeffect transistor (A), NO gas sensor (B) and photodetector (C).


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RESULTS AND DISCUSSION The TaS3 nanofibers were synthesized by vapor phase growth method. Firstly, the morphological and structural characterizations were performed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in combination with energydispersive X-ray spectroscopy (EDS), precession electron diffraction tomography and X-ray powder diffraction (XRD). Chemical composition was further verified by high-resolution Xray photoelectron spectroscopy (XPS). SEM images (Figure 2) show long nanofibers with length of over 500 µm (Figure 2A and B). An individual fiber with a typical size over 100 nm in diameter and lamellar structure characteristic of layered compounds is clearly visible in Figure 2C. The lamellar fibers also undergo a lateral aggregation such that fibers with micron size diameters can be found (Figure 2D and E).


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Figure 2. The SEM images of TaS3 nanofibers with various magnifications (A) 1 kx, (B) 5kx, (C) 100 kx and 25 kx (D and E).

The high-resolution XPS spectra of Ta 4f and S 2p were evaluated to study the chemical state and bonding of TaS3 nanofibers (Figure 3A). The deconvoluted XPS spectra of the Ta 4f region revealed two main peaks at 26.12 and 27.95 eV. This indicated that the Fermi level was located at the top of the valence band, demonstrating the metallic nature of TaS3.19 Nevertheless, the two peaks deconvoluted (161.53 and 162.79 eV) from the spectra of S 2p region demonstrated the presence of polysulfides.20 In this way, the successful synthesis of the metallic TaS3 nanofibers was demonstrated.


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Figure 3. (A) High resolution XPS spectra of TaS3 nanofibers for Ta 4f and S 2p regions. (B) Powder X-ray diffraction pattern of TaS3 fitted on C-TaS3 phase model.

The structure was also investigated in detail by EDT. Both P- and C-TaS3 phases were observed but the decisive majority of the nanoribbons were those of the C-phase. Both


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structures were solved using Jana200621 and Superflip22 software packages and they were subsequently dynamically refined23 using Dyngo software. The ribbons of P-TaS3 were without any significant disorder or twinning. The 0kl, h0l and hk0 sections through reciprocal space are shown in SI (Figure S2). The refined structure (R(obs) = 7.05 %, GOF(obs) = 2.10 for more details see cif file in SI P-TaS3.cif) was similar to the published model (average and maximal difference between atomic positions were 0.009 Å and 0.025 Å, respectively). The elemental composition of the crystal from energy dispersive X-ray spectroscopy (EDS) is shown in Table S1 together with EDS spectrum (Figure S3). Ni, Fe and Au are signals from microscope and TEM grid. The resulting Ta:S ~ 1:2 ratio (standardless) is typical for all measured crystals. We suppose that this deviation is caused by the absence of standardization (since an excellent structural match with P21/m structure was achieved). It is important to note, that the tips of the fibers always showed an increased oxygen content in comparison with the rest of the fibers. A part of this oxygen signal also comes from the oxidized carbon foil of the TEM grid. The slightly increased concentration of oxygen observed at the ends of TaS3 nanofibers indicates its important role in the growth of nanofibers, however, no caps or regions with different structure typically observed for vapor–liquid–solid (VLS) grown nanowires were found on any TaS3 sample. The structure viewed along b direction (growth direction of the nanofibers) is shown in Figure S4. The formal valence of Ta is +4.67 as there are four S22- groups in a unit cell. The average Ta-S bond length in the TaS8 bicapped trigonal prisms is 2.544 Å. The interactions between the layers are much weaker. The shortest interlayer Ta-S bonds are between 4 and 5 Å (Figure S4). As mentioned in the introduction, nearly all crystals were of the C-TaS3 phase. In comparison with the nearly defect-less crystals of the P-phase the nanofibers of the C-TaS3 usually


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contained lots of defects, mainly twinning by 180° rotation along a* axis. We may observe this twinning as a violation of C-centering in for example l = 2n+1 rows in h0l reciprocal space section (Figure S5b) and also in other directions (Figure S5a and S5c). In case of extensive twinning, streaks of diffuse scattering were observed along a* direction (Figure S5e) as well as other directions (Figure S5d and S5f). The streaks were successfully modelled using program FAULTS24 and the results are shown in Figure S5g. This direction corresponds to the layer stacking in the P-phase. During the survey of the sample by TEM it was possible to find a few non-twinned crystals with nearly no diffuse scattering streaks (Figure S6). EDS results yield again the ratio Ta:S~1:2. The symmetry of the unit cell is monoclinic C2/m with the a lattice parameter doubled compared to the P21/m phase (a = 19.9(3) Å, b = 3.34(4) Å, c = 15.2(2) Å, beta = 112.4(5)°, for more details see SI_PB CTaS3.cif). The transformation of the unit cell axes derived by Bjerkelund25 is a = 0.5aBj 0.5cBj, b = cBj, and c = bBj. The structure of the C-phase was successfully solved and refined with Jana2006, Superflip and Dyngo programs. The C-centering is apparent from the hk0 section through the reciprocal space (Figure S6). The structure viewed along b direction is shown in Figure 4. The structural change within a layer of the TaS8 prisms is relatively subtle. The number of the S22- groups in a unit cell is eight, which is the same number as in the P21/m because Z is doubled due to C-centering. Thus, the formal Ta charge is again +4.67. The structural change is a mirror operation of a part of the TaS8 chain, which forms the P21/m structure as shown in Figure 4. This subtle structural change has a large impact on the layer stacking sequence because from the point of the local symmetry the monoclinic layer stacking along a is equivalent to the layer stacking along a + c (Figure S7). Therefore, it is not surprising that the majority of the crystals of the C-phase are twinned. The layered stacking is clearly visible on HR-TEM images (Figure 5) showing the structure view from (010) direction.


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Figure 4. Structure of C-TaS3 phase. Red Ta atoms y = 0.00, green Ta atoms y = 0.50. The structural change, which differentiates the P-phase from the C-phase is illustrated in the upper right corner. To obtain the layer found in P21/m phase one has to mirror a part of the layer through the blue color highlighted mirror plane.

Figure 5. TEM (A, B and C) images of TaS3 nanofibers and HR-TEM (D) image of TaS3 with clearly visible (200) and (006) planes.

The powder X-ray diffraction (PXRD) data were fitted with the C2/m structure model. The model described the data satisfactorily (a = 19.914(5) Å, b = 3.337(3) Å, c = 15.167(5) Å, β = 11 ACS Paragon Plus Environment

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112.4(4)°) after the preferential orientation was refined and broader profile for hkl diffractions with l = 2n+1 was allowed (Figure 3B). These diffraction maxima suffer from twinning. Introduction of the P-phase into the model did not improve the resulting fit. This is in accord with TEM observation where the P-phase was observed only in a few cases.

Figure 6. (A) The electronic band structure of TaS3 in the C2/m form. The Fermi level is set to zero of the energy and X=(a*/2,0,0), Y=(0,b*/2,0), Z=(0,0,c*/2). (B) The temperature dependence of resistivity of TaS3 nanofibers.

The electronic structure calculations of the C-TaS3 (Figure 6A) show the one-dimensional metallic conductivity manifested by eight dispersive bands cross the Fermi level along the Γ to Y direction. These bands are predominantly based on the Ta dz2 orbitals. The dz2 character of the bands at the Fermi level is similar to TaS2, dichalcogenide counterpart of TaS3.26 Along the Γ to X and Γ to Z direction, all bands are very weakly dispersive and the Fermi level lies


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in the pseudogap. Overall, the bandstructure of C2/m TaS3 is very similar to the band structure of the monoclinic P21/m TaS3 calculated by Canadell et al.27 Although the electronic structure calculation shows the one-dimensional metallic character of TaS3, strong temperature dependence of resistivity was observed. The resistivity increased in the range of 7 orders from room temperature up to 30 K. The results are shown in Figure 6B. The increase in resistivity in TaS3 below 210 K has been explained in terms of metalsemiconducting transition induced by the CDW formation.14 The mechanism of CDW formation in transition metal trichalcogenides has been discussed in several studies.15,

16, 28

TaS3 undergoes a Peierls transition typical for one-dimensional metals with high density of states at Fermi level (Fermi surface nesting). As a result of Peierls instability a periodic modulation of the electron density referred to as a CDW is established, accompanied by opening of a band gap and structure distortion. Our resistivity measurement indicates this kind of metal-semiconductor transition manifested by an anomaly in resistivity slightly above 200 K followed by a sharp increase in resistivity below this temperature (Figure 6B). The C2/m and P21/m structures were also used for calculation of phonon spectra and simulation of Raman spectra which were correlated with experimental results (Figure 7). Despite similarities in structural and electronic properties, theoretical spectra of both structures of TaS3 are markedly different. The spectrum of C2/m shows dominant Raman peak at 299 cm-1, which is easily distinguishable in the experimental spectrum. Another footprint of C2/m seems to be the peak at 483 cm-1, which is present in the experimental spectrum but completely missing in the spectrum of P21/m (there is no vibrational mode of P21/m having the wavenumber higher than 400 cm-1). On the other hand, the spectrum of P21/m contains three peaks in the range 300-400 cm-1, which can be found in the experimental spectrum and which indicate admixture of P21/m structure to the dominant C2/m phase in experimental samples.


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Figure 7. Measured (A) and simulated (B) Raman spectra of TaS3.

Subsequently, the nitric oxide (NO) sensing capability of the TaS3 nanofibers was evaluated using electrochemical impedance spectroscopy (EIS).3,4 For this purpose, NO was produced by adding 2 mM H2SO4 into a saturated solution of NaNO2, previously purged with nitrogen. The concentration of NO in saturated stock solution was 1.8 µM at 25 °C.29 TaS3 nanofibers were attached onto three different substrates (polyester, parafilm and paper) with the aid of double-sided tape and for the electrochemical measurements, silver ink was applied at both ends of the TaS3 fibers (see Figure S1 and experimental section in the supporting information). The changes in electronic properties of TaS3 upon adsorption of NO 16.2 µM were probed using electrochemical impedance spectroscopy (Figure 8A). In all cases the impedance module grew by one (paper) or two (polyester and parafilm) orders of magnitude in comparison with the impedance module measured in air. Control experiments were performed to confirm that the impedance response observed was related only to interactions between the TaS3 nanofibers and NO (Figure S8 A, B and C in SI). For this purpose, water was injected instead of H2SO4 into the NaNO2 saturated solution and the TaS3 sensors were exposed to this environment. After 30 min, no significant responses were observed. 14 ACS Paragon Plus Environment

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Figure 8. (A) Experimental (dots) and simulated (continuous line) of impedance module of TaS3 gas sensor in the presence of 16.2 µM NO gas. The TaS3 sensor was fabricated on polyester, parafilm and paper substrates, the inset shows the equivalent electrical circuit (EEC) used for modeling the impedance module. (B) Impedance responses of TaS3 sensor fabricated in polyester, parafilm and paper under different NO concentrations from 0 to 16.2 µM. (C) The impedance phase responses of the gas sensor fabricated in different substrates


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(polyester, parafilm and paper) as functions of NO concentration. (D) Calibration curves of the TaS3 gas sensor fabricated in polyester and paper under different NO concentrations. (E) Selectivity study comparison of the TaS3 gas sensor fabricated in all configurations (paper, parafilm and polyester). An equivalent electrical circuit (EEC) (see inset in the Figure 8A) was proposed for modelling the structure of TaS3 sensor. The EEC includes three electric circuits composed of a parallel resistor–capacitor (RC) placed in series with two resistors at both terminals corresponding to the resistance of the silver ink (RAg). The RC circuit placed behind the RAg represents the interface between the silver ink and the TaS3 nanofibers (CAg/TaS3 and R Ag/TaS3) while the last RC circuit (placed between the circuits of the Ag/TaS3 interface) corresponds to the interface between the TaS3 nanofibers (CTaS3 and RTaS3).

Table 1. Passive elements from the equivalent electrical circuit used for modelling the impedance module of the metallic TaS3 gas sensor in the presence of NO gas. The TaS3 was fabricated using 3 different substrates (polyester, parafilm and paper). Substrate

RAg (kΩ)

RAg/TaS3 (kΩ)

CAg/TaS3 (nF)

RTaS3 (kΩ)

CTaS3 (nF)

Polyester

4.58

16.9

0.26

212

3.70

Parafilm

5.77

177

0.42

33.3

0.03

Paper

0.19

0.32

108

0.17

10.1

Table 1 summarizes the values of the passive elements obtained from the EEC of the TaS3 gas sensor in the presence of NO 16.2 µM. The RAg was similar in parafilm and polyester based sensor. For the paper based sensor, RAg was smaller, which is due to a better adhesion of silver ink to paper device. To confirm this assertion, control experiments were performed, silver ink was used instead of TaS3 nanofibers and the impedance measurement was done in the presence of 16.2 µM NO. As observed, no responses were recorded in all the silver sensors (see Figure S8 D, E and F in the SI). In the Ag/TaS3 interface, C component was higher in the paper based sensor, while in parafilm and polyester based sensors this 16 ACS Paragon Plus Environment

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component was similar but much smaller than for the paper based sensor. Nevertheless, R component in the Ag/TaS3 interface was the lowest for the paper based device. In TaS3 nanofibers the R and C of parafilm based sensor decreased by about one order of magnitude when compared to the R and C values of in the Ag/TaS3 interface on both polyester and parafilm. The paper based sensor showed a different trend with the C component of the TaS3 nanofibers decreasing by a factor of ten. Therefore, the different responses observed at the different substrates were due to the change in the tensile strain of the metallic TaS3 nanofibers in each substrate and consequently, the change in total impedance due to the introduction of defects and dislocations within the TaS3 nanofibers. Such defects were indeed observed from TEM and SEM images (Figure S9). Afterwards, the impedance phase was evaluated and was chosen as a sensing signal, because when the concentration of the NO increased, the impedance phase value increased at a certain frequency referred to as a resonant frequency (fr) (see Figure 8B). This NO gas sensor showed a fast response time ~15 min being comparable with its counterpart TaS2.18 However, different gas sensing performances were observed for each substrate as was expected from the study of the EEC components proposed to model the structure of the sensor. The parafilm based sensor showed a higher sensitivity at NO concentrations bellow 10.8 µM (fr=2.51 kHz) but when the NO concentration was increased the impedance phase decreased, while for the paper and polyester based sensor the impedance phase grew consistently with increasing NO concentration over the wide range from 1.8 to 16.2 µM (see the summarized responses of sensors in Figure 8C). The fr for paper and polyester based sensor were 1.26 and 25.12 kHz respectively. Measurements in unpurged NaNO2 solution were also performed yielding lower impedance responses in all cases (see Figure S10 in SI). In this way, we confirmed the selective response of TaS3 nanofibers to NO, since NO is quickly oxidized to NO2 in the presence of air.


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Subsequently, the analytical performance of the TaS3 gas sensors fabricated on paper and polyester substrates were evaluated, since they showed the most favorable analytical parameters (Figure 8D). Table 2 summarizes the linear range, limit of detection (LOD) and limit of quantification (LOQ), correlation coefficient (r) and sensitivity.

Table 2. Analytical performance of TaS3 gas sensor fabricated using different substrates. Substrate Sensitivity (O/µM)

Linear range (µM)

r

LOD (µM)

LOQ (µM)

Polyester

4.48

5.4-14.4

0.97

0.016

0.054

Paper

1.35

1.8-16.2

0.99

0.241

0.804

The polyester based sensor showed superior performance in terms of sensitivity, LOD and LOQ than the paper based sensor. This can be attributed to higher rigidity of polyester compared to paper, offering a better support and avoiding the introduction of defects and dislocations within the TaS3 nanofibers. Since the polyester based sensor shows a better performance, the reproducibility was evaluated measuring the impedance response of three TaS3 gas sensors constructed onto this substrate and were exposed to 12.6 µM NO concentration. The impedance phase average obtained was 34.67±6.48 and reasonable reproducibility was observed with a relative standard deviation (RSD) value of 18.7%. Furthermore, the lower LOD obtained for paper and polyester based gas sensor of 0.241 µM=7.2 ppb and 0.016 µM=0.48 ppb respectively, are seven (paper) and hundred (polyester) times lower than the allowed value set by environmental agencies.6, 17 On the other hand, the LODs obtained for these gas sensor are much lower if we compare it with the other transitional metal chalcogenides like MoS2 and TaS2 used for detection of NO, where they have shown LODs of 800 ppb and 190 ppb, respectively.18, 30


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The selectivity performance was also evaluated using the most common interferences that are present with 16.2 µM NO during fuel combustion. The TaS3 gas sensor in all configurations (paper, parafilm and polyester) does not show any impedance response after the exposition of N2, CO2, benzene, and water humidity as shown in Figures 8E and S11 in SI.

TaS3 can be also utilized in nanoelectronic devices. For this purpose we investigated individual nanofibers as a material for construction of optoelectronic detectors and field effect transistor. The images of TaS3 nanofibers used for FET device together with electrodes are shown in Figure 9A and the FET transistor characteristic curves are represented in Figure 9B. Moreover, nanofibers used for light detector and its photodetector characteristic curves are shown in Figure 9C and Figure 9D respectively. A typical DC output characteristic of “freestanding” TaS3 nanofiber based FET with a 15µm gate length is presented in Figure 9A. Our FET device exhibits ~ 0.3 mA maximal drain current. Compared to TaS3, the family of transition metal dichalcogenides is substantially more explored. Within this group, semiconductors (e.g. MoS2) as well as metals (e.g. TaS2) can be found. TaS2 is well known for its properties like low temperature superconductivity, charge density wave formation and topological phases formation.31-37 The TaS2 was investigated in detail for possible construction of devices based on charge density wave formation and Mott insulator phase transitions, however to our knowledge no reports on FET device and photodetector fabrication based on TaS2 have been published.38-42 Our observation on different nanowires structures reveal, however, only a moderate variation in the resistance measured (about +/-10%) depending on the degree/angle of the individual integrated nanostructures characterized in this study. Coplanar strip-lines based design was also used for a construction of photodetectors (Figure 9C). Nanofibers structures were first tested by DC measurements in the dark and under


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continuous wave (cw) 405 nm illumination with 100 µW average laser light input power. Current-voltage measurements performed on contact-annealed TaS3 nanofiber structure exhibit ohmic dependence in the range from ~0,6 V up to 10 V and their responsivity shows quadratic dependence at higher biases (see Figure 9D). The quadratic dependence can be explained by well-known space-charge effects resulting from the application of a very high electric field in the presence of recombination limited transport. The observed responsivity of TaS3 based photodetectors is in the range of responsivities of classical semiconductors like silicon43,44 and AIIIBV semiconductors.45-47

Figure 9. The optical image of TaS3 fiber in FET transistor structure (A) and FET transistor characteristic curves (B) at different Vg (gate bias) of -1 V (violet curve), 0 V (blue curve), 1 V (green curve), 2 V(red curve) and 3 V(black curve). Optical image of TaS3 nanofiber photodetector using gold contact lines (C) and its electrical characteristic curves (D) that including dark current (dark curve) and its responsivity (red curve).

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We synthesized C-TaS3 nanofibers whose structure was solved and refined using electron precession diffraction tomography. The C-phase is a one-dimensional metallic material, and its band structure is similar to that of the known P-TaS3. The calculated Raman spectra demonstrated that P- and C-phase could be distinguished based on their Raman patterns. Raman spectroscopy measurements are in good agreement with theoretical calculations. Despite the one-dimensional metallic character of the prepared layered trichalcogenide, the resistivity thermal dependence indicated metal-semiconducting transition below 210 K induced by the charge density wave formation. TaS3 nanofibers were used for the construction of various devices including FET transistor, photodetector and NO gas sensor. The prepared photodetector exhibits excellent responsivity comparable with traditionally used materials based on AIIIBV and Si semiconductors. The transistor characteristic was measured in the Vg range of -1 to +3 V showing suitability of TaS3 nanofiber for FET device fabrication. The impedimetric gas sensors exhibit excellent responsivity towards nitrogen oxide (NO). Consequently, three different substrates (polyester, parafilm and paper) were used for TaS3 sensor fabrication in a resistor configuration. Different responses were observed in each device developed due to the different tensile strain of the C-TaS3 nanofibers. The TaS3 nanofibers supported by a polyester substrate exhibited excellent selectivity, sensitivity and linear range properties towards nitrogen oxide (NO) sensing whose control is crucial in environmental protection applications. Moreover, this NO gas sensor based TaS3 shows enhanced analytical performance in terms of sensitivity (LOD=0.48 ppb) in comparison with the NO gas sensor developed using theTaS2 congener.18 EXPERIMENTAL SECTION Materials Tantalum (99.9%) and sulfur (99.999%) were obtained from STREM (France). Sodium nitrite and sulfuric acid were obtained from Sigma-Aldrich (Singapore).


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Synthesis Synthesis was performed in evacuated ampoule from elements at elevated temperature. 1.74 g of sulfur and 3.26 g of Ta were placed in quartz glass ampoule and melt sealed by oxygenhydrogen torch under vacuum (below 5x10-3 Pa). The ampoule of size 20x150 mm filled with precursors was placed in muffle furnace in horizontal position and heated on 500 °C for 48 hours. Characterizations Morphology studies were measured by scanning electron microscope (SEM) equipped with FEG source of electron (Tescan Lyra dual beam microscope). Before the measurement, samples were placed onto a conductive carbon tape to avoid charging effects. All the measurements were carried out with 15 kV acceleration voltage. Electron diffraction experiments were performed on a Phillips CM120 microscope (120 kV) with a LaB6 cathode equipped with an Olympus SIS Veleta CCD camera (14 bit) and a Digistar precession electron diffraction (PED) unit. Tils step was 1° and precession angles was also 1°. High resolution transmission electron microscopy (HR-TEM) was carried out on a transmission electron microscope JEOL JEM 2200FS operated at 200 kV (auto emission Shottky gun, point resolution 0.19 nm) with an in-column energy Ω-filter for EELS/EFTEM analyses, a STEM unit, and Energy Dispersive X-ray (EDX) SDD detector Oxford Instruments X-Max attached. Images were recorded on a Gatan CCD camera with resolution 2048×2048 pixels using the Digital Micrograph software package. EDX analyses were acquired and treated in the INCA software package. X-ray photoelectron spectroscopy was performed with a Phoibos 100 spectrometer and a monochromatic Mg X-ray radiation source (SPECS, Germany). Both wide survey and highresolution spectra for S 2s and Ta 4f were collected. Relative sensitivity factors were used for


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the evaluation of atomic Ta/S ratios from XPS wide survey spectra measurements. XPS samples were prepared by coating a carbon tape with a uniform layer of the materials under study. X-ray diffraction was performed with CuKα radiation (λ = 0.15418 nm, rotating anode) using Rigaku SmartLab diffractometer. Sample was measured in glass capillary. Data were analyzed using Fullprof software package.

Gas sensing setup To demonstrate the potential application of metallic TaS3 nanofibers as a NO gas sensor we first attached the nanofibers (~0.2 mg) to substrates (paper, polyester or parafilm) by doublesided adhesive tape and Ag ink was coated at both ends of the fibers (see Figure S1 in the SI). For the NO gas sensing, saturated NaNO2 (from sigma Aldrich, Singapore) solution was placed into the glass chamber and the TaS3 sensor was placed and attached to the chamber cover. Subsequently, the system was purged with N2 for 20 min and H2SO4 was injected in the NaNO2 solution to obtain a final concentration of 1.8 µM. The concentration of NO in these conditions will be 1.8 µM at 25 °C. Impedance measurements were carried out by applying a sinusoidal potential modulation of ±10 mV amplitude in a frequency range of 0.1 Hz to 100 kHz, using an Autolab PGSTAT 204/ FRA 32M (Eco Chemie, Utrecht, The Netherlands) controlled by NOVA version 1.1 software (Eco Chemie).

FET and photodetector device fabrication Individual TaS3 nanofibers were separated under microscope and placed on host substrate. The entire technological process was described in detail in our previous work for the fabrication of micrometer sized picosecond and femtosecond photo-switches.48,49 We


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improved the micromanipulation of the nanofibers structures and increased the effectiveness of the fabrication processes by using a micrometer-sized pipette of quartz glass, which allows precise manipulation of our nanometer/micrometer sized structures. After nanofibers transfer to the host substrate, the structures were positioned with 5-10µm accuracy on the desired location and bonded through van der Waals forces to the coplanar strip lines (for photodetectors) and 3 terminal metallic electrodes (Ti/Au, 10/300nm) fabricated on sapphire substrate (for transistor). The entire structure was subsequently annealed with the help of a focused laser (He-Cd, 325 nm) beam to improve the electrical contact and mechanical stability as well as crystallinity of the transferred TaS3 nanofiber and to eliminate possible defects on their sidewalls. The entire procedure - laser micro annealing (LMA) was already described in our previous work.50 Theoretical Calculations DFT calculations were performed using the projector-augmented wave method implemented in the Vienna Ab initio Simulation Package (VASP).51,52 The energy cutoff for the plane-wave expansion was set to 350 eV. In order to account for weak van der Waals bonding between layers, we used optimized van der Waals functional optB86b-vdW.53 We utilized 3x8x3 kpoint grid for structural optimization and band structure calculation. The bands were interpolated using the Wannier90 package.54 For calculations of the vibrational properties, we utilized density functional perturbation theory approach implemented in VASP. The calculations were done at the calculated lattice constants of each respective phase. Offresonance Raman activity of vibrational modes was calculated by evaluating the derivative of the polarizability (or macroscopic dielectric tensor) with respect to that mode’s coordinate using Raman-sc package. The calculated line spectra were broadened by a Lorentzian resolution function with a width of 5 cm-1.


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ASSOCIATED CONTENT Supporting Information. The images of fabricated gas sensors; SAED images for various crystallographic orientations; elemental composition obtained by EDS in TEM; structural drawing of TaS3 phases; curves of control experiments and selectivity for NO gas sensing; SEM and TEM images of defects within TaS3. CIF files for the P- and C-phases. AUTHOR INFORMATION Corresponding Author *[email protected] (M. Pumera). *[email protected] (Z. Sofer). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGEMENTS Z.S., Š.H., D.S. and J.L. thank for financial support from specific university research (MSMT No 20-SVV/2017) and Czech Science Foundation (GACR No. 17-11456S and GACR No. 1605167S). This work was created with the financial support of the Neuron Foundation for science support. P. L. acknowledges support from the Ministry of Education, Youth and Sports of the Czech Republic (project LO1305). This work was supported by the project Advanced Functional Nanorobots (reg. No. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR). References 1.

Yang, W.; Gan, L.; Li, H.; Zhai, T., Two-Dimensional Layered Nanomaterials for Gas-Sensing Applications. Inorg. Chem. Front. 2016, 3, 433-451. 25 ACS Paragon Plus Environment

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TaS3 Nanofibers: Layered Trichalcogenide for High-Performance

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