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Towards the Realization of 2D Borophene Based Gas Sensor Vivekanand Shukla, John Wärnå, Naresh K Jena, Anton Grigoriev, and Rajeev Ahuja J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09552 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 11, 2017

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The Journal of Physical Chemistry

Towards the Realization of 2D Borophene Based Gas Sensor Vivekanand Shuklaa, John Wärnåa, Naresh K. Jena*a, Anton Grigorieva and Rajeev Ahuja*a,b a

Condensed Matter Theory Group, Materials Theory Division, Department of Physics and Astronomy, Uppsala University, Box 516, SE-75120, Uppsala Sweden b Applied Materials Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH), SE-10044, Stockholm Sweden *Corresponding author email: NKJ([email protected] ; [email protected]); RA([email protected])

ABSTRACT

To the league of rapidly expanding 2D materials, borophene is a recent addition. Herein, a combination of ab initio density functional theory (DFT) and non-equilibrium Green’s function (NEGF) based methods is used to estimate this prospects of this promising elemental 2D material for gas sensing applications. We note that the binding of target gas molecules such as CO, NO, NO2, NH3 and CO2 are quite strong on the borophene surface. Interestingly, our computed binding energies are far stronger than several other reported 2D materials like graphene, MoS2 and phosphorene. Further rationalization of stronger binding is made with the help of charge transfer analysis. The sensitivity of the borophene for these gases is also interpreted in terms of computing the vibrational spectra of the adsorbed gases on top of borophene, which show dramatic shift from their gas phase reference values. Metallic nature of borophene enables us to devise a setup considering the same substrate as electrodes. From the computation of the transmission function of system (gas + borophene), appreciable change in the transmission functions are noted compared to pristine borophene surface. The measurements of currentvoltage (I-V) characteristics unambiguously demonstrate the presence and absence of gas molecules (acting as ON and OFF states), strengthening the plausibility of a borophene based gas sensing device. As we extol the extraordinary sensitivity of borophene, we assert that this elemental 2D material is likely to attract subsequent interest.

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Introduction From the prototypical example of graphene as a two-dimensional (2D) materials we have

learnt that reduced dimensionality manifests exciting realms of physical and chemical phenomena. Ultra-high Fermi velocity, massless Dirac fermion, superior electron and carrier mobility are concepts that came into vogue and continues to vividly inspire recent scientific pursuits1,2. Following this exemplary path of graphene, several other elemental 2D materials came in to practical realization and few pertinent examples include silicene3,4, stanene5, germanene6, phosphorene7 etc. In addition, transition metal dichalcogenides (TMDCs) also represent another emerging class of 2D materials8–10. In general, 2D materials possess great promise for futuristic nano-device applications based on their outstanding mobility in single layer, strong mechanical nature and high surface to volume ratio. From device point of view, the main drawback in case of graphene was an absence of band gap which makes it difficult to control the charge in the sheet2. Similarly, form the family of TMDCs, MoS2 shows low mobility3. All these shortcomings kept pushing the scientific world to search for new kind of 2D materials which can circumvent such problems and can improve the performance. Recently, single layered boron has successfully been synthesized on the Silver surface under ultra-high vacuum conditions and it is termed as borophene11–13. Interestingly, similar structure was also proposed from first principles density functional theory (DFT) calculations14. There are also reports which discuss the hydrogenation of borophene leading to a new material namely borophane that behaves as a Dirac material analogous to the graphene15. In a curiosity driven study, this borophane has been explored as a potential 2D material for battery applications16. It is known that boron has electronic configuration of 2s22p1 and bond between B-B is very complex having several polymorphisms12. It is noteworthy that 2D borophene exhibits anisotropic metallic properties which differs from graphene having a zero band gap and this metallic nature of borophene was predicted theoretically before the synthesis15. This recent report of realization of borophene, thus, has offered new possibilities to be explored in terms of its practical applicability. 2D materials are promising candidates for gas sensing applications due to their large surface to volume ratio and associated charge transfer from gas to surface. Gas sensor based on graphene has drawn more attention because of good sensing ability of graphene for various

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gases, low electronic temperature noise, high chemical stability and fast response time17,18. Graphene has been extensively studied both in theory and experiment, for gas sensing applications. Several other 2D surfaces have also been studied for gas detection applications such as silicene, MoS2, Phosphorene etc19–21. Within a short span of time after its report, borophene has gained considerable attraction particularly from the point of view of a probable anode material for Li and Na ion batteries22–24. In a recent study, Peng et al. have carried out a first principle study of the optical and electronic properties of borophene and underlined the anisotropy in the band structure (anisotropic metal) and optical properties25. Likewise, the directional dependence of transport properties has been addressed by Padilha et al. using DFT methods15. Indeed, there are many other promising frontiers where this emerging 2D material can find application, one of such area is a substrate for gas adsorption and detection, which we aim to delve into. Boron based nanostructure B40 fullerene has already been studied for NH3 at very low bias26. This is important to understand the interaction between the borophene monolayer and adsorbate gas molecules to fully understand of the possibilities of borophene based sensing device. In this manuscript, we report our first principle DFT based calculations for several gas molecules CO, CO2, NO, NO2 and NH3 on monolayer borophene. We investigated their preferable orientation, arrangement and corresponding binding energies. Our results suggest that strength of binding depends upon the amount of charge transfer between gas molecule and borophene monolayer. Keeping other 2D materials such as phosphorene, graphene and MoS2 in perspective, our results show superior binding of gases on to borophene so do the amount of charge transfer indicating better sensitivity for the later. Further, from the point of view of a device, we computed the transmission functions and corresponding I-V traces for gas adsorbed on host borophene with the aid of Non-equilibrium Green’s Function (NEGF) formalism. Our results reveal that gas adsorption makes appreciable changes to the transmission functions and IV characteristics which can form the basis of a realistic device with distinct ON and OFF states. 2.

Computational Methods First principle density functional theory (DFT) calculations were performed using 27

VASP (Vienna ab initio simulation package) within generalized gradient approximation (GGA) with Perdew, Bruke and Ernazerholf (PBE) functional28. We took care of van der Waals (vdW) corrections using Grimme (DFT-D3) method29.

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The plane wave cut off was 500 eV and



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Brillouin zone integration was sampled by 8x6x1 K-points within Monkhrast-pack scheme30. Projected augmented wave (PAW) potentials were used to describe the ion-electron interaction31. The systems were fully relaxed to obtain the ground state structure with residual forces on the atoms less than 0.01 eV/atom. The charge transfer analysis due to adsorption of gases on borophene is done using Bader charge method as implemented in VASP.32 Additionally, to estimate the change in vibrational properties associated with the gas binding process, we have employed hybrid functional B3LYP with 6-31G(d) basis sets with dispersion corrections (GD3) in the Gaussian code.33 For the Gaussian calculations, small cluster models of borophane (Figure S2 in the Supporting Information) have been considered which are relaxed independently with tight convergence criteria. From these calculations, we have also computed the vibrational spectra for the bare gases and the gas + borophene systems. Such an analysis will yield more chemical insights about the binding process of the gases on borophene. The electronic transport properties have been studied using non-equilibrium Green’s function (NEGF) in Transiesta module of Siesta34,35 code. For this purpose, we again independently relaxed the gas+borophene systems in Siesta with an energy cut-off of 200 Ry and using double-ς polarized (DZP) basis sets. For Siesta calculations, however, we have also considered the van der Waals interactions (Grimme) and all these calculations are not spin-polarized. We performed transport calculations to determine the sensing capability of pristine borophene monolayer as nano-device to distinguish the 5 different molecular gases (CO, NO, NO2, NO2 and NH3). We, however, tested the effect of spin polarization for Siesta calculations for one specific case (for paramagnetic gas NO) and it is observed that there is no significant effect of spin to the transport properties. Valence electrons were described using local basis set with single-ς polarized (SZP) basis sizes and 1x24x16 k-points grid have been used during the transport calculations. Our choice of such basis sets is based on our previous experience with other 2D systems.36 System was divided in to three parts as left and right lead and central scattering region. A more detailed description of the electronic transport methods can be found elsewhere35. The electric current through the scattering region is calculated using Landauer-Buttiker formalism as,

= , − − −

(1)

Here T (E, Vb) is transmission probability of electrons incident at potential energy Vb and G0 is the unit of quantum conductance. µL and µR are the electrostatic potentials of left of right

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electrode at a particular voltage bias. It is important to note that our discussions of the electronic properties of borophene+gas system is based on our results on VASP calculations whereas the purpose of employing Siesta and Gaussian programs are only meant for computations of transport properties and vibrational properties, respectively. 3.

Results and Discussions To begin with, we relaxed the unit cell of the borophene and obtained the lattice constants

(a, b) as 1.64 x 2.90 Å, which is in good agreement with previously published result15. In the next step, we modeled the borophene monolayer in a 5x4 supercell as shown in Fig.1 with lattice parameter of 8.10 x 11.73 Å. The structure of borophene is anisotropic along X and Y directions which are labeled as linear and zigzag directions. Subsequently, the adsorption behavior of different gases such as CO, CO2, NO, NO2 and NH3 on borophene was investigated. In the case of diatomic gases like CO and NO, we tried all possible orientations of molecules with molecular axis parallel and perpendicular with respect to borophene surface. For triatomic gases viz. CO2, NO2 and tetratomic gas NH3, two possible orientations have been investigated one with N and C down facing borophene and their inverted counterparts. The most stable adsorption configurations for the adsorbed gas molecules on monolayer borophene are shown in Figure 1(aj). Let us analyze the bonding of the gases with the borophene surface more carefully. CO shows stronger bond character and C atom binds to surface with a C-B distance of 1.49 Å and the angle between surface and gas (B-C-O) of 171.2o (Figure 1a). The bond length of C-O in free standing CO molecule is 1.14 Å and it becomes 1.16 Å on binding with the surface. In the case of CO2, the shortest distance between the gas and surface is 1.82 Å and the bond between C and O in CO2 molecule is 1.18 Å compared to 1.15 Å in its isolated form. In the case of NO, the bonding distance with the surface is 1.44 Å with N-O bond length of 1.21 Å that is larger than in its isolated form (1.17 Å). Similarly, NO makes an angle of 143.8o with the surface. NO2 is a paramagnetic gas which has strong electron withdrawing character. NO2 is also chemisorbed on the borophene monolayer and the shortest distance between gas and surface is 1.51 Å. For the adsorbed NO2, the N-O bond distance is 1.27 Å which is slightly higher than that of isolated form (1.22 Å). We find O-N-O bond angle to be 124o which is significantly smaller than what is observed in bare NO2 (140o). Moving on to the tetratomic gas NH3, the shortest bonding distance from the surface is found to be 1.63 Å with N atom directly bonding with B-atom of the 5



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borophene. Considering the N-H bond distance (1.04 Å) for the surface adsorbed NH3, there is no appreciable difference from its free counterpart. Keeping all the bonding pictures of different gases on borophene surface, it is apparent that the nature of interaction is predominantly chemisorption with significant bonding involved. This will be more clear from our subsequent discussions of the binding energies.

For the quantitative description of the adsorption strengths on the borophene surface, adsorption energies (Ead) have been calculated which is defined as the energy of fully relaxed borophene with the gas on the surface (EB+Egas) minus the energy of the pristine borophene layer (EB) and the energy of isolated gas molecules (Egas). According to this definition, a negative Ead value indicates that the adsorption of gas molecules on the borophene surface is energetically favorable. Adsorption energies for different gas molecules along with the distance from the surfaces have been summarized in Table 1. It is also evident from these values that the interactions of CO, NO, NO2 and NH3 are relatively stronger and chemisorbed in nature. On the other hand, the bonding for CO2 is attributed to be weak physisorbed in nature. CO2 has weakest adsorption on borophene surface while NO2 has the largest adsorption energy among all the gases studied here (Table 1). The binding energy values for CO, NO and NH3 lie in between. For the sake of comparison, we have also computed the binding energies of borophene+gas systems from Siesta and Gaussian calculations, although these two programs have been exclusively used for the purpose of NEGF calculations and vibrational properties, respectively. The binding energies obtained by Siesta and Gaussian program are listed in Table 2 alongside the VASP results. These values clearly suggest that the trends in binding energies like weakest interaction for CO2 and strongest for NO2 are reasonably well captured irrespective of the use of different programs. Moreover, the overall trend also remains consistent irrespective of the choice of the code. On subsequent discussions, we however focus on our primary results from VASP calculations. All these adsorption energies (except CO2) for the borophene surface are large enough to be affected by thermal disturbance at the room temperature which is in the energy scale of KBT (KB is the Boltzmann constant). This indicates that borophene can be an ideal surface for gas detection at room temperature form the point of view of practical utility. It is noted that most of the gases showed higher adsorption energies to the borophene compared to several other reported 2D materials. We have summarized the adsorption energies of these gases

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on different 2D surfaces as obtained from the literature along with borophene in Table 2. Graphene, for instance, shows weaker binding energies for small gases in the range of meVs18,37. Adsorption energies reported for the different gases on MoS2 surface are slightly higher with 0.44, -0.33, -0.55 eV for CO, CO2, NO, respectively38. Similarly, for another important member from the 2D material family, silicene exhibits higher adsorption energies as available from the literature such as -0.18, -0.35, -1.37, -0.04, -0.60 eV for CO, NO, NO2, CO2 and NH3, respectively39.

Analogous qualitative trend of gas adsorption is also observed for 2D

germanene.40 Therefore, if we consider these values keeping our results in perspective, we find that our adsorption energies are appreciably higher for most of the cases. Hence, the above discussion justifies the sensitive nature of borophene substrate towards favorable gas sensing.

For the better understating the adsorption properties of different gases on the borophene surface, we have analyzed the charge density difference which is defined as ∆ = − − ! . Where ρtot (r) is charge distribution on borophene along with the adsorbed gases, ρB(r) is the charge distribution on borophene and ρgas(r) is the charge distribution on the isolated gas. Conceptually, ∆ρ depicts the charge accumulation/depletion in the system by which we can quantify the charge transfer. The charge density figures as presented in Figure 2, gives us a visual clue about the stronger bonding interaction of the gases (except for CO2) on the borophene surface. Further, we have computed the charge transfer (Bader charges) associated with the binding of gases on borophene. The total charge on the adsorbed gas molecules (in units of e) are presented in Table 1. Among all gases, the weakest interacting species (CO2) manifests a negligible charge transfer of -0.09 which is also consistent with its binding efficiency.

The electron withdrawing gases NO and NO2 are associated with significant

withdrawal of charge from the surface as evidenced from the net charge values of -0.62 and 0.72, respectively.

Similarly, we find that CO molecule also withdraws electron from the

substrate having a net charge of -0.39. The gas NH3, on the other hand, is found to be giving away electrons with a net charge of +0.18. This appreciable amount of charge-transfer observed for gas-borophene systems indicate the sensitive nature of borophene substrate for gas detection. Furthermore, finer chemical intuition associated with the gas adsorption is obtained from the computation of vibrational frequencies of bare gases and borophene + gas systems. From the comparison of key vibrational frequencies of free and adsorbed gas molecules, we can judge the 7



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strength of interaction of the gases. For the CO-borophene system, the stretching frequency is computed to be 2158 cm-1 which is red-shifted from its free counterpart (2209 cm-1). For CO2 molecule, the symmetric stretching and asymmetric stretching of the gas-borophene system (free gas) are 1370 cm-1 (1365 cm-1) and 2418 cm-1 (2434 cm-1), respectively. This result clearly reveals the weak interacting nature of CO2 that we have already inferred from the binding energy and charge transfer calculations. For NO, the stronger influence of bonding can be judged from the appreciable redshift of N-O stretch from 1991 cm-1 in the free gas to 1581 cm-1 in the adsorbed form. In an analogous spirit, for NO2 the asymmetric stretch changes from 1719 cm-1 in the free molecule to 1579 cm-1 upon bonding with the surface. For polyatomic NH3, there can be many possible (3N-6=6; N=number of atoms) normal modes. However, simply considering the case symmetric stretch to illustrate our case, we see similar trend (3435 cm-1 in free gas vs 3355 cm-1 in adsorbed form) as with other interacting gas molecules. Therefore, from the above discussion we infer about the favorable chemical bonding interactions (except CO2) of gas molecules on borophene substrate which induces significant changes in their vibration properties. This change in the vibrational properties can also be a criterion for gas detection. However, our emphasis is on detecting the gases from the change in transmission function and I-V characteristics which we will discuss in the forthcoming lines. We are going to delve more into the influence of gas molecule adsorption on the electronic properties of borophene. Total density of states (DOS) of pristine borophene monolayer and borophene with adsorbed gases are shown in Figure 3(a-e) along with the projected density of states (PDOS) on the gases for all five cases. Our results show metallic behavior of borophene sheet as inferred from the absence of a band-gap, which is completely in agreement with previous published results25,41. Among all the gases CO2 is reported to have lowest binding energy. For CO2, there is no appreciable influence of the gas adsorption on borophene electronic structure which is completely in agreement with weak adsorption energy and small charge transfer picture for CO2 molecule. However, for all remaining gases we can see that these molecules induce changes in the DOS (broaden the peak) particularly around Fermi level, which can be easily inferred by comparing with the DOS of pristine borophene. Moving on to the gas CO, peak around 1 eV indicates that there is charge transfer from surface to the gas molecule. Broadening of this peak is due to the hybridization of LUMO level of gas molecule

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with the states of borophene. Similarly, in the case of NH3, we also find hybridization of states around Fermi leading to broadening of DOS. The gases NO and NO2 are paramagnetic in nature. For NO, withdrawal of electron from borophene monolayer can be understood from Figure 3(e) by looking at the peak broadening around the Fermi level on PDOS for NO. This is in good agreement with our binding efficiency, vibrational properties and charge transfer picture that we have so far discussed. We have different PDOS for up and down spins in NO case, which also shows magnetic moments on N as 0.3 µB and on O as 0.3 µB. This magnetization occurs because of the presence of unpaired electron in NO. The case of NO2 adsorption on borophene deserves special attention as this gas manifests highest binding energy and the paramagnetic gas (isolated case) is changed to nonmagnetic due to binding with the substrate. In case of NO2, the states below Fermi level are hybridized with Boron p states and make it nonmagnetic. This is clear from figure S3 (SI) where we present PDOS variation as this molecule is lifted from its relaxed bonding configuration (nonmagnetic) to a non-bonding configuration (magnetic). Based on all the discussions so far, it is pertinent to shed more light on the possibility of borophene based device set up for gas sensing. Although, we observe some influence on the electronic structure around the Fermi level for some of the gas molecules, the charge transfer induced by adsorption of these gases on borophene can produce different sensitivity for resistive measurements in the experiments. To understand the performance of borophene as a good sensor in resistive measurements, non-equilibrium Green’s function (NEGF) method is employed to calculate the transport properties and corresponding current-voltage characteristics of borophene before and after the gas adsorption. Obtained results can be used to make qualitative comparison to experiments. Figure 4 shows the set up for transport calculation where shaded areas represent left and right electrodes. Central region is the scattering region where the different gases were adsorbed. It has been already reported that borophene monolayer has anisotropic transport behavior7. We considered the transport along the linear (X) direction because it is reported to transmit higher current values in this direction. Reported current ratio in the linear and zigzag direction (n=Ix/Iy) is 2.115. Zero bias transmission function for bare borophene monolayer and borophene with different gases have been presented in Figure 5. Noticeable effect of gas adsorption in the transmission spectra can be observed. Adsorption of gas molecule results in decreased transmission which can be attributed to back scattering which inhibits the available

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conduction channels. CO induces small change in transmission function whereas CO2 has negligible effect in the transmission function. In the case of paramagnetic gases, we have charge transfer from monolayer to the gas molecule. Accordingly, we see that for NO, the change in the transmission function form the pristine borophene is appreciably larger. This shift is also in good agreement with the shift in the vibrational frequencies considering the pristine borophene and borophene + NO system, that we already discussed. Analogous trends can also be observed for NO2 and NH3. Hence, from the perspective of designing a sensor, either from monitoring the change in transmission function or shift in the vibrational frequencies borophene can be particularly important for N-containing gases such as NO, NO2 and NH3. In order to design a borophene based sensing device where gas detection can be accomplished from the corresponding current-voltage (I-V) characteristics, it worthwhile to compute the I-V traces. In Figure 6, I-V characteristics of all gas-borophene system have been summarized. Borophene monolayer shows Ohmic behavior in its current properties15. The above figure presents I-V trends of all gases up to 1 V. When such a borophene based device is exposed to gases, the current drops down as compared to the case of pristine borophene. This decline in current is due to the difference in the resistances as offered by different gases. Of special mention is the case of borophene-CO2 that shows no variation in the current compared to the reference of pristine borophene.

This observation is reminiscent of the weak bonding

features of this gas with the substrate which we have already discussed. For the case of paramagnetic gas NO, we have already discussed the effect of spin polarization on the DOS in the previous sections. To further check the effect of spin polarization to transport properties, we computed spin-polarized I-V as presented in figure S4 in SI. It is noted that both the spin channels manifest almost identical I-V trends. This is due to the fact that although there is some effect of spin polarization on PDOS of this molecule, this effect is quite negligible if we compare the total DOS (which is primarily dominated by substrate borophene) for both the spin channels. Another way of looking at Figure 6 is to plot the resistance (V/I) versus current which is presented in figure S1 in the Supporting Information (SI). From this figure, clearer distinction between gases can be made. In particular, NO and NO2 offer different resistance and can be clearly distinguishable. Further, CO and NH3 behave similarly, whereas the trace for CO2 is almost indistinguishable with the pristine borophene. There is increase in resistance on the adsorption of the gas molecules (CO, NO, NO2 and NH3) which is manifested in terms of decline

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of the current signals. Typically, the gas molecules inhibit some of the conductance channels by introducing backscattering of the electrons. Viewing from the perspective of designing an effect device for gas sensing, we feel that borophene can be an ideal substrate. The presence and absence of gases which is clearly manifested in terms of distinct current traces can be regarded as ON and OFF states for this device. The sizeable difference in the current signals on account of gas adsorption can be the basis of a superior borophene based sensing device with better sensitivity.

In this connection, it is important to note the performance of some other 2D

materials reported earlier for gas-sensing. For instance, in case of 2D phosphorene based gas sensor, distinction between current traces for different gases starts at ~ 1.2 V which is considered to be much higher. 20 Similarly, for a graphene based device, clear distinction comes out to be at 0.6 V for NO2 and 0.8 V for NH342. On the contrary, in 2D borophene we start getting differences in current signals at smaller biases. Moreover, the magnitude of the current is also appreciably high for better detection. For example, in case of NO, the difference in current values between the pristine and gas-adsorbed systems at a bias voltage of 1V amounts to nearly 100 µA. Hence, all these unique features make 2D borophene an attractive material for devising a realistic sensor. 4.

Conclusion In summary, with an aim to widen the scope of applications of recently reported 2D

material borophene, its prospects as a gas sensor is computationally explored. Single or few atomic layer thickness, improved stability and availability of larger surface area are some of the key virtues which make 2D materials so attractive for gas sensing. In this work, using first principle density functional theory methods, we investigated the electronic and transport properties of borophene monolayer with adsorption of several gas molecules such as CO, NO, CO2, NO2 and NH3. Our results suggest that the binding energies for all the gases are appreciably higher except for CO2, which shows lower adsorption energy. We have further corroborated the adsorption behaviors in terms of charge transfer analysis and computations of vibrational spectra for borophene + gas systems. Envisioning a sensing device with electrical detection of gases, we have further computed the transmission functions and I-V characteristics for borophene + gas systems with pristine borophene as a reference. Detectable quenching of transmission and I-V features (compared to the case of pristine borophene) as a result of gas adsorption is observed

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which can be regarded as OFF and ON states of the sensing mechanism. The notable exception, however, is CO2 where no appreciable change in transmission or current is registered. Based on all these finding, it is apparent that 2D borophene can be a potentially important substrate for gas sensing and our study demonstrates the possibility of designing a borophene based gas sensor nanodevice. Having fundamental ramifications, the emergence of borophene as a promising elemental 2D material with excitingly new applications is likely to attract further attention.

Supporting Information The Supporting Information contains further details corresponding to resistance versus voltage profiles for borophene + gas systems, and optimized geometries of the borophene cluster with the adsorbed gases used for obtaining vibrational spectra.

Acknowledgements The authors gratefully acknowledge the Swedish National Infrastructure for Computing (SNIC) at National Supercomputer Centre (Triolith) and HPC2N (Abisko). VS acknowledges the European Erasmus fellowship program for funding. RA acknowledge support from the Swedish Research Council (VR), Carl Tryggers Stiftelse for Vetenskaplig Forskning (CTS) and StandUP.

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Table 1. Calculated binding energies (Eb), shortest bonding distance (D), and net (Bader) charges on the gas molecules for different gas + borophene systems Adsorbed Gas on borophene CO CO2 NO NO2 NH3

Eb (eV)

D (Å)

Net charge on gas (e)

-1.38 -0.36 -1.79 -2.32 -1.75

1.49 1.82 1.44 1.51 1.63

-0.39 -0.09 -0.62 -0.72 +0.18

Table 2: Comparative summary of adsorption of gases (adsorption energies in eV) on different 2D surfaces collected from literature against on the borophene surface. Values in bracket indicate binding energies computed from a siesta and b Gaussian program. Gas Surface Borophene Graphene18,37 Silicene39 Phosphorene20 Germanene40 MoS238,43

CO

CO2

NO2

-1.38 (-1.71a, 1.32 b) -0.01 -0.18 -0.32 -0.16 -0.44

-0.36 (-0.83 a , -0.30 b) -0.05 -0.04 -0.41 -0.10 -0.33

-2.32(-3.35 a, 3.32 b) -0.07 -1.37 -0.60 -1.08 -0.14

13


NO -1.79(-2.00 a, 2.15 b) -0.03 -0.35 -0.86 -0.51 -0.55

NH3 -1.75(-1.81 a, 2.03 b) -0.03 -0.60 -0.50 -0.44 -0.16


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

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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


(a)

(f)

CO

(b)

NO

(c)

(g)

NO2

(h)

(d) CO2

(e)

(i)

(j)

NH3

Figure1. Top (a)-(g) and side (f)-(j) view of relaxed structures of borophene monolayer with adsorbed gases CO, NO, NO2, CO2 and NH3. Purple balls represent boron atoms, while gray, red, cyan and brown balls represent N, O, H and C atoms, respectively.

(a)

(b)

(d)

(c)

(e)

Figure 2: Charge density difference figures for gas adsorption (a)-(e) (CO, NO, CO2, NO2, NH3 respectively) on borophene surface. (Isosurface value is 0.003 e/Å3; red surface indicates electron gain, while the blue surface represents electron loss)



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

(a)

(b)

(d)

Page 18 of 20

(c)

(e)

Figure 3: Total DOS (black) and projected DOS for (a)-(e) borophene + gas systems (CO, CO2, NH3, NO2, NO respectively). As a reference, DOS for the pristine borophene surface is presented as dashed lines. Fermi level is shifted to zero in each case. Electrode

Scattering Region

Electrode

Figure 4: An illustration of the device set up (top and side views) showing the semi-infinite left and right electrode regions and the central scattering region.


Page 19 of 20

17 16 15

Borophene+CO2 Borophene+CO Borophene+NO Borophene+NO

2

Borophene+NH Transmission (G/G0)

14

3

Borophene

13 12 11 10 9 8 7 −2

−1.5

−1

−0.5

0 Energy (eV)

0.5

1

1.5

2

Figure 5: Zero bias transmission for pristine borophene and borophene + gas systems.

600 Borophene Borophene+CO Borophene+NO Borophene+NO2 Borophene+NH3 Borophene+CO2

500

400 Current (µA)

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


300

200

100

0

0

0.1

0.2

0.3

0.4

0.5 0.6 Voltage (V)

0.7

0.8

0.9

1

Figure 6: I-V characteristics for borophene monolayer along with different adsorbed gas molecules



TOC Graphic

Borophene (OFF) Borophene+Gas (ON)

-4

5×10

-4

4×10

Current (A)

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

3×10

-4

2×10

-4

1×10

0

0

0.1

0.2

0.3

0.4

0.5 0.6 Voltage (V)

0.7

0.8

0.9

1


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Toward the Realization of 2D Borophene Based Gas Sensor - The

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