Semiconductor-like Sensitivity in Metallic Ultrathin Gold Nanowire

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Semiconductor-like Sensitivity in Metallic Ultrathin Gold NanowireBased Sensors Ahin Roy, Tribhuwan Pandey, N. Ravishankar, and Abhishek K. Singh* Materials Research Centre, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: Due to the ease of modification of electronic structure upon analyte adsorption, semiconductors have been the preferred materials as chemical sensors. At reduced dimension, however, the sensitivity of semiconductor-based sensors deteriorates significantly due to passivation, and often by increased band gap caused by quantum confinement. Using first-principles density functional theory combined with Boltzmann transport calculations, we demonstrate semiconductor-like sensitivity toward chemical species in ultrathin gold nanowires (AuNWs). The sensing mechanism is governed by the modification of the electronic structure of the AuNW as well as scattering of the charge carriers by analyte adsorption. Most importantly, the sensitivity exhibits a linear relationship with the electron affinities of the respective analytes. Based on this relationship, we propose an empirical parameter, which can predict an analyte-specific sensitivity of a AuNW, rendering them as effective sensors for a wide range of chemical analytes.

L

competing effects, which affects overall reliability of these sensors, hindering their prospect in practical applications. Single crystalline metal nanowires, being free from such defects, could overcome this shortcoming of polycrystalline wires and have potential to be better sensors. One such candidate is experimentally fabricated, [111] oriented, ultrathin single crystalline gold nanowires42 formed by oriented attachment of crystalline Au nanoparticles. These wires are of ∼2 nm diameter and are free of high angle/high energy grain boundaries, minimizing the interface scattering, and can have interesting electronic properties.43,44 Here, using first-principles calculations, we systematically explore the possibility of using [111] oriented ultrathin single crystalline AuNW as a potential sensor for detection of various analytes such as CH3S, CO, CH3NH2, and CH3CHO. We find that the nanowire responds with a significant change in conductance on adsorption of analytes. Our calculations reveal that the sensitivity of the AuNW strongly depends on electronic interaction of the analyte with the nanowire. In fact, the sensitivity of AuNW toward analytes is greater than that of Si nanowires toward pH,45 or single-walled CNTs for electronically interacting analytes.46 Furthermore, we show that the sensitivity depends linearly on the electron affinities of the analytes thereby providing unprecedented predictability in detection of a wide range of analytes using these AuNW.

ow-dimensional materials have emerged as an exciting class of sensors due to enhanced sensitivity caused by the quantum confinement of the carriers and increased surface area.1−7 Carbon nanotubes (CNT), CNT-based heterostructures,8−11 carbon nanofibers, and semiconducting nanoparticles/wires are considered as most promising sensors due to their excellent electrical response to analyte exposure. Until recently, most of the research has been carried out on semiconducting nanowires,1,12−24 where the sensing is mediated by altering the space charge region. However, an increase in band gap at nanoscale due to quantum confinement in semiconducting nanowires leads to insulating behavior, which is undesirable for sensing. On the other hand, noble metal nanowires, being free from such hindrances, are ideal systems to detect changes in conductance caused by the adsorption of chemical species. Metal nanostructures are not new to sensing, for example, hydrogen has been detected with electrodeposited Pd25,26 and Pt nanowires27 with lithographically patterned electrodes. Silver nanowires have been used for ammonia sensing.28 Due to biocompatibility of Au, Au-based nanostructures have been used for biosensing.29−32 Numerous reports have illustrated the formation of self-assembled monolayer 33−39 of various molecules on gold surfaces. The sensitive dependence of conductance of ∼10 nm gold nanowire (AuNW) on molecular adsorption has been probed experimentally, which suggested the possibility of detection of a single molecule.40 In all these cases, the wires were polycrystalline in nature, where the conductance is not only affected by the adsorption of analyte, but also is limited by the grain boundary scattering in the nanowire.41 Often, it is very hard to separate these two © 2014 American Chemical Society

Received: January 5, 2014 Revised: July 15, 2014 Published: July 24, 2014 18676

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Figure 1. Relaxed cross-sectional geometries of the AuNW after adsorption of (a) CH3S, (b) CO, (c) CH3NH2, and (d) CH3CHO; panels e−g represent three nonequivalent adsorption sites, namely, edge, atop1, and atop2, respectively; panel h shows the angle (θ) between AuNW and analyte.

Table 1. Binding Energies, Bond Lengths, and Angle between the Analyte and AuNW for the Three Symmetrically Nonequivalent Sites Eb (eV)

a

system (bond)

edge

atop1

atop2

{110} surface

reported ({111} surface)

bond length (Å)a

δ (Å)

θ (deg)

CH3S(Au−S) CO(Au−C) CH3NH2(Au−N) CH3CHO(Au−O)

−2.39 −1.01 −0.99 −0.44

−2.58 −0.67 −0.85 −0.3

−2.65 −0.75 −0.87 −0.29

−2.83 −0.81 −0.76 −0.22

−1.43 −0.0451 −0.5352 not reported

2.3 1.96 2.28 2.47

0.03 0.04 0.02 0.02

105.9 175.1 114.9 42.68

50

This work.

The calculations were performed by using first-principles density functional theory. The ionic cores are described by allelectron projector augmented wave potentials47,48 and the Perdew−Burke−Ernzerhof49 generalized gradient approximation to the electronic exchange and correlation as implemented in the Vienna Ab Initio Simulation Package (VASP) code.47,48 Au is a heavy element and, therefore, the scalar relativistic effects are incorporated in the PAW pseudopotential. Geometries were considered as converged when the component of interatomic forces was less than 0.005 eV/Å. All integrations over the one-dimensional irreducible Brillouin zone were performed by using a Monkhorst−Pack 1 × 1 × 5 k-grid. To ensure the accuracy of the results, the kinetic energy cutoff for the plane wave basis set was 300 eV. The dimension of the unit cell along the longitudinal direction (periodic direction) was fully optimized and found to be 7.17 Å. In each calculation, more than 12 Å vacuum space was included in the transverse directions to avoid interaction among the periodic replicas of the systems. The number of Au atoms in the unit cell was 61. Crystalline AuNW, oriented along the [111] direction was generated by cleaving bulk Au with the low index surfaces. The wire was terminated with six {110} surfaces, which essentially leads to a hexagonal cross-section, as shown by Figure 1a−d. The cell length along the nanowire axis and the atomic positions in the unit cell have been optimized without any symmetry constraint. The analytes were adsorbed on the wires at several symmetrically nonequivalent sites and completely relaxed. The adsorption sites, designated as edge, atop1, and atop2, are shown in Figure 1e−g. The edge site is located along

the line of intersection of two {110} surfaces, whereas the two other atop sites are the two symmetrically nonequivalent positions on the {110} surface. The adsorption of analytes on AuNW at different adsorption sites is determined by their respective binding energies. This binding energy (Eb) is defined as E b = Eanalyte + wire − Ewire − Eanalyte

(1)

where Eanalyte+wire, Ewire, and Eanalyte are the energies of the analyte adsorbed AuNW, of the analyte-free AuNW, and of the analyte, respectively. From the minimum energy configuration, the preferential binding site of the analyte was determined and used for further analysis. Relaxed cross-sectional geometries of the AuNW after adsorption of analytes are shown in Figure 1a− d. The binding of the analytes to AuNWs is much stronger than on the bare Au surfaces, as can be seen from Table 1. This is partially caused by the large surface to volume ratio of nanowire compared with the bulk surfaces. In the case of the surface (as the surface to volume ratio is lower compared to the nanowire) the electronic effects of the analyte binding are not felt in the interior of the crystal, but are restricted to the few terminating layers of the slab. There also exists a trend in the binding strength. Both {110} surface and AuNW show a similar trend in analyte binding. This clearly shows that the stronger the withdrawal capacity (higher electron affinity), the higher the binding. In the case of an ultrathin nanowire, the electronic structure of the entire system is perturbed because of adsorption, and hence the binding also becomes stronger. 18677

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soft ligands more strongly. Going from left to right of the above series, the softness of the analyte decreases and hence reduces the binding. This is consistent with the trend in the electron withdrawing capacities of the analytes. In Table 1 we have added the binding energies on the {111} surface. However, the trend is not clear from those values. This clearly depicts that the sesitivity would depend on the nature of the bounding surfaces. The adsorbed analytes make an angle (θ) (e.g., ∠Au−S−C for the Au−CH3S system) as shown in Figure 1h. The value of θ is highest for CO followed by CH3S, CH3NH2, and CH3CHO as shown in Table 1. This trend is solely governed by the steric effect. A higher value of q implies the freedom of the analyte to move into different configurations. As the size of the analyte increases, it tends to confine itself to a rigid geometry due to steric hindrance of free rotation around the Au−X (X is the docking site of the analyte, e.g., S of CH3S) bond leading to a decrease in θ. Structurally, we observe an axial displacement of atoms in the analyte-free relaxed nanowire.43,53 The analyte adsorption leads to further displacement of the Au atoms in the nanowire from their relaxed positions. In this regard, CH3S was found to have the maximum effect (see Figure S1 in the Supporting Information), whereas CH3NH2, CO, and CH3CHO were found to have very little effect. Interestingly, the bond lengths in molecules significantly deviate from their pristine state. For CH3S, CH3NH2, and CH3CHO, the bond between the linking atom of the analyte and the carbon of the adjacent methyl group is weakened after adsorption. The same happens in the case of the C−O bond. The slackening of the bonds has been listed in Table 1 as δ. This also indicates a back-donation of d-electrons from Au into antibonding orbitals of the ligands leading to a decrease in bond strength within the analyte. This reflects in the d-band shift of Au atoms to lower energy after the adsorption of analyte on AuNWs as shown in the density of states (Figure 2). The CO adsorption shows the maximum shift followed by CH3S, CH3NH2, and CH3CHO. This clearly shows a one-to-one relationship between the extent of the hybridization and the weakening of the bond in the analyte. The total DOS calculation shows a decrease in electron density of states near the Fermi level (EF) upon adsorption of the analytes on the AuNW. From this decrease, it is difficult to conclude about the mechanism of electron withdrawal. On the other hand, the l-decomposed adsorption site projected DOS (PDOS) shows a very clear trend of electron withdrawal from the valence orbital, namely from the s-orbital of the Au atoms. The l- and site-decomposed DOS are calculated by projecting the electronic wave function of the system onto the spherical harmonics in the sphere around the Au atom on which the analyte is adsorbed. Au, being a transition metal with half-filled

Moreover, due to a lower dimensionality of the nanowire compared to a surface, the carrier confinement is stronger and hence the binding as well as sensitivity to the analytes would also be greater. We found that CH3S gets adsorbed with a 2fold binding with atop2 and atop3 sites (Figure 3), whereas all the other analytes prefer to bind on the edge site. The electronwithdrawing analytes like CH3S and CO exhibit high binding energies that originate from the strong hybridization with available d-electrons in the AuNW. This can be seen in the adsorption-site-projected partial DOS (Figure 2), where the Au

Figure 2. l-decomposed adsorption site projected electronic density of states: the left panel shows the analyte molecules. s (before), p (before), and d (before) denote s, p, and d decomposed DOS projected on the adsorption site in AuNW prior to adsorption; s (after), p (after), and d (after) represent s, p, and d decomposed DOS after adsorption.

d-band shifts significantly toward lower energy upon the analyte adsorption. On the other hand, electron-donating analytes such as CH3NH2 and CH3CHO show slightly lower binding energies due to reduced preference of AuNW to accept the electrons. Therefore, as expected, the strength of the binding exhibits the following trend: CH3S > CO > CH3NH2 > CH3CHO. Furthermore, from the hard−soft acid base (HSAB) principle, which relates polarizability of chemical species to their binding tendencies, it is expected that soft Au would bind

Figure 3. Charge accumulation depletion plots for analyte adsorbed wires: (a) CH3S, (b) CO, (c) CH3NH2, and (d) CH3CHO. Red and blue respectively denote depletion and accumulation of electron density. For all cases, the isosurface level was set to 0.005 e/Å3. 18678

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Figure 4. (a) Charge transfer obtained from Bader charge analysis. CH3S and CO withdraw an electron from AuNW, whereas CH3NH2 and CH3CHO donate an electron to AuNW; the dashed line denotes charge on the pristine AuNW. (b) Conductivity of AuNW and analyte adsorbed AuNW at a low temperature range; (c) conductivity of AuNW and analyte adsorbed wires in the room temperature range.

outermost orbital, can donate an electron to attain a stable d10 configuration. Hence, the withdrawal of electrons from the sorbital is expected to be much more pronounced compared to the p or d states. The l-decomposed adsorption site projected DOS, Figure 2, shows that the highest s-electron withdrawal near the Fermi level happens when thiol is adsorbed on the AuNWs, followed by the CO, amine, and aldehyde, respectively. This trend of the electron withdrawal follows the electron affinity of the respective analyte species. To obtain a better understanding of the electron withdrawal/ donation interaction of the analytes with the AuNW, adsorption-induced electron density redistribution was calculated as shown in Figure 3a−d. Clearly, maximum electron depletion occurs at the anchoring Au site, which follows the trend CH3S > CO > CH3NH2 > CH3CHO, in fact we observe an electron density accumulation at the anchor site for CH3NH2 and CH3CHO. Such electron density redistribution is indicative of the electron-withdrawing or -donating capacity of the adsorbed analyte from AuNW. A more quantitative estimate using Bader charge analysis was carried out,54−56 which showed a significant electron transfer between wire and adsorbed molecule. From Figure 4a, it is clear that thiol and CO withdraw an electron from AuNW whereas aldehyde and amine donate an electron. This trend is consistent with the magnitude of experimentally measured electron affinities of CH3S, CO, CH3NH2, and CH3CHO, which are 1.86,57 1.32,58 0.56,57 and 0.003659 eV, respectively. On the basis of the charge analysis and electronic structure alone, it is difficult to explain the efficiency of these wires as sensors. As the quality of a sensor depends on its electrical transport in response to adsorption, it is worthwhile to estimate the changes in conductivities. The conductivity not only depends on the electron transfer but also on the scattering of conduction electrons upon adsorption of the analyte. As the surface area to volume ratio becomes increasingly large at nanoscale, scattering effects start to dominate. Conductivity also depends on the changes in electronic structure caused by analyte adsorption. To investigate the effect of different adsorbed analytes on transport properties, the electrical conductivity of AuNW is calculated by using Boltzmann Transport theory (BTT) within constant scattering time approximation (CSTA),60,61 as implemented in the BoltzTraP program.62 In CSTA, it is assumed that the scattering time τ determining the electrical conductivity does not vary strongly with temperature. In the BTT, the motion of an electron is treated semiclassically. The group velocity of an electron in a particular band can be described as

να(i , k) =

1 ∂ε(i , k) ℏ ∂k α

(2)

where ε(i,k) is the ith energy band at point k, and kα is the α component of wavevector k. From group velocity να(i,k), the conductivity tensor can be obtained as e2 N

σαβ(i , k) =

∑ τ(i , k)να(i , k)νβ(i , k)δ[ε − ε(i , k)] (3)

where e, τ(i,k), α, and β are elementary charge, the relaxation time, and the Cartesian indices, respectively. N is the total number of k points sampled. The electrical conductivity tensor is also given by an equivalent equation: σαβ(T , μ) =

1 V

∫

⎡ ∂f (T , ε) ⎤ μ ⎥ dε σαβ(ε)⎢ − ∂ε ⎢⎣ ⎥⎦

(4)

where V and fμ are the volume of the unit cell and the Fermi Dirac distribution function, respectively. For calculation of transport properties, the group velocities can be obtained by Fourier interpolation63 of the band energies as a function of k. With use of these values, the transport properties and Fermi integrals as given in eq 4 are calculated. This approach has been shown to provide a good estimate of transport properties for a variety of materials.64−66 From the electronic structure it is possible to calculate σ/τ as a function of temperature (T) and carrier concentration (n), but it is not possible to calculate σ without relaxation time (τ). For conductivity calculation the Drude model relaxation time is used.61 To test the reliability of our calculations, we ran a benchmark calculation for bulk gold. The conductivity of bulk Au at room temperature is found to be 1.10 × 107 Ω−1 m−1, which agrees well with experimentally reported value of 4.90 × 107 Ω−1 m−1.61 To estimate the transport property of AuNW, the electronic structure calculations were performed at a denser grid of 1 × 1 × 40. The calculated electrical conductivity of all AuNW is plotted as a function of temperature and shown in Figure 4b,c. As expected, the bare AuNW shows metallic conductivity, which decreases with increasing temperature as shown in Figure 4b. In all the cases, the conductivity of analyte-adsorbed AuNW remains much below the conductivity of pristine AuNW. The trend in conductivity behavior can be explained on the basis of density of states as shown in Figure 2. As explained in the previous section, the interaction of the analyte with AuNW influences the s-states of AuNW near Fermi energy, which is reflected in the conductivity trend. We have summarized the 18679

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nanowires toward pH (130%),45 or single-walled CNTs for electron withdrawing (NO2) or donating (NH3) analytes (50− 80%).46 Most importantly, based on γ, given the electron affinity of an analyte (χm), conductivity (σm) and hence the sensitivity can be predicted. Such systematic dependence provides a rational basis for the design of effective sensors for different analytes. In summary, for the first time we show that single-crystalline AuNW can serve as a potential nanoscale sensor for chemical species with a semiconductor-like sensitivity. The main factors which affect the conductivity of the analyte-adsorbed nanowires are (i) the electron transfer between NW and analyte and (ii) the number of electronic states near the Fermi level. Sensitivity of AuNW shows a systematic variation with the electron affinities of respective analytes, furnishing them as an effective sensor for a range of molecules. This concept introduces a roadmap for an ultrathin AuNW based device for efficient detection of chemical species. The rich chemistry of Au, combined with the biocompatibility could also lead to applications in biosensing.

effect in a schematic in Figure 5b. The contribution of Au s orbitals near EF as well as electron transfer from the analyte to

Figure 5. (a) Dependence of the device sensitivity on the electron affinities of analytes. σp and σm represent the conductivity of pristine AuNW and analyte adsorbed AuNW, respectively. (b) Schematic showing the mechanism of conductivity modification. σanalyte+wire denotes the conductivity of the analyte adsorbed AuNW, DOS(s) denotes the Au s-orbital contribution near EF after adsorption, and ΔCTanalyte−wire is the charge transfer from the analyte to AuNW.

■

Details of atomic displacements caused by analyte adsorption and details of the method of adsorption calculation on the Au {110} slab and the relaxed structures of the analytes adsorbed on the slab. This material is available free of charge via the Internet at http://pubs.acs.org.

the AuNW goes up with decreasing electron affinity, which in turn modifies the conductivity of the system. Thus, we show that the overall effect of the conductivity is governed by electron transfer and electronic structure modification. Since adsorbed molecule affects the conductivity of AuNWs, the sensitivity of an experimental device based on such a nanowire can be measured by the change in conductivity: Δσ = (σp − σm), where σp and σm are conductivities of pristine wire and analyte adsorbed wire, respectively. Clearly, sensitivity of the AuNWs is increased in the presence of different analytes in the order of σwire+CH3S > σwire+CO > σwire+CH3NH2 > σwire+CH3CHO. To investigate the tuning of conductance with molecular adsorption, we fitted an empirical parameter, ln(σp/σm), with the electron affinity as shown in Figure 5a. This parameter scales linearly with the electron affinity of the analyte species. The slope of this linear fit is given by

γ=−

1 ⎛ σp ⎞ ln⎜ ⎟ χm ⎝ σm ⎠

ASSOCIATED CONTENT

S Supporting Information *

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the Supercomputer Education and Research Centre and Materials Research Centre, IISc, for providing the required computational facilities for the above work. Financial support from the ADA under NPMASS and DST nanomission is gratefully acknowledged.

■

(5)

where χm, σp, and σm are the electron affinity of the analyte and conductance of the pristine and analyte-adsorbed AuNW, respectively. Clearly, an analyte with large electron affinity will have large binding energy and stronger electronic interaction. Therefore, this will cause significant changes in conductivity resulting in higher sensitivity. Such behavior of tuning electrical conductivity by the electron affinity of adsorbed species has been reported before for micrographitc carbon fibers.67 We observe a similar trend for the metallic gold nanowire. For comparison, the value of the γ for the micrographitic-carbonbased sensors for different molecules is of the order of 20 eV−1.67 Our study shows a very high value of γ, 33 eV−1, which makes ultrathin single crystalline AuNW a promising sensor. Furthermore, γ also has very little temperature dependence, as it is directly proportional to σp/σm, which is weakly dependent on temperature. Very recently, Kisner et al. have experimentally studied alkanethiol adsorption on AuNW,68 which shows a ΔR/R of 180%, which is in excellent agreement with our calculations. In fact, the sensitivity of AuNW toward strongly interacting analyte (180% for CH3S) is greater than that of Si

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