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Conductance-Based Chemical Sensing in Metallic Nanowires and Metal-Semiconductor Nanostructures Conductance-based chemical sensing in metal-semiconductor nanostructures and all-metal nanowires of atomic dimensions is garnering increased interest. Adsorbed gas molecules can migrate to a metal-semiconductor junction, thereby shifting the magnitude of the Schottky barrier and altering electrical impedance, whereas atomic scale metal junctions can sensitively report the presence of adsorbates through their impact on ballistic electron transport. Barrett K. Duan,† Jingying Zhang,‡ and Paul W. Bohn*,†,‡ † ‡
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States Beyond thin film gas sensors, the physical morphology presented by nanomaterials, such as nanowires, nanotubes, nanorods, and nanopores, can extend the utility of these structures in chemical sensing, both by presenting a higher surface-to-volume ratio than planar crystals and also by enabling new physical phenomena, such as quantum conductance. Nanostructured metal-semiconductor junctions are of interest, because the Schottky barrier that forms at specific metal-semiconductor interfaces can be exploited for gas sensing. Gaseous molecules may adsorb and diffuse, either intact or as atomic constituents, to the M-S interface, where they can affect the magnitude of the Schottky barrier, ϕS, thereby altering the electrical impedance of the structure.7 Beyond this rather welldeveloped sensing strategy, all-metallic nanostructures also exhibit conductance-based sensing properties. At first, this may seem surprising: after all, the vast majority of semiconductor-based nanomaterial sensors work through analyte-induced modulation of the space-charge region, and the miniscule Thomas-Fermi screening length in metals, typically ∼0.1 nm,8 mitigates against this mechanism in metallic nanowires. However, it has been known since the 1930s9 that the surface affects resistivity disproportionately for metal films thinner than the electron mean free path, λe (ca. 80 nm for Au),10 and surface adsorption, especially of Lewis bases, has been shown to dramatically alter overall conductance in nanoscale metal structures.11,12 Recently, it has become possible to study these types of conductors at their physical limits by constructing junctions composed of small arrays of atoms or molecules. Atom-scale junctions (ASJs) composed entirely of metal atoms have been studied since the mid-1990s,1315 and their conduction properties were interpreted within the Landauer formulation, which interprets ballistic electron transport as transmission between two perfect, but energetically offset, reservoirs.16 While some studies of atom-scale junctions have addressed molecular adsorption,17,18 most have focused on the preparation of molecule-sized gaps and the subsequent study of single molecule conductance,1921 the subject of recent comprehensive reviews.22
Robert Gates
C
onductance-based chemical sensing in semiconducting nanostructures, especially nanowires, has attracted a great deal of attention. In this Feature, we address metallic and metalsemiconductor (M-S) nanostructures, structures that possess attractive features for chemical sensing but which have received less attention as chemical sensors than semiconductor nanowires. Studies of chemically mediated electrical conduction in nanowires and ultrasmall junctions can be traced back to studies of electrical conduction in metal oxide films for detection of gasphase contaminants in hostile environments,1,2 and the use of organic3,4 or inorganic5,6 semiconducting nanowires, where sensing is mediated by altering the space charge region (SCR). r 2011 American Chemical Society
Published: August 12, 2011 2
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Figure 1. Schematic diagram illustrating the relationship of the interfacial layer to band bending and width of the SCR. Untreated Pt-GaN has an interfacial polarization and an n-type depletion layer, leaving an electron deficient SCR. In the presence of adsorbed H atoms, the interfacial dipole density is increased, and the barrier height, ϕ, and SCR width, W, are decreased.
This review addresses both of these less commonly encountered sensing nanostructures, and the remainder of this article addresses the use of metal-semiconductor Schottky barriers and metallic nanowires for chemical sensing. Special attention is paid to understanding sensing mechanisms, nanofabrication strategies, and prospects for future applications.
Figure 2. Fabrication strategy used by Penner and co-workers to develop Pd mesowire arrays for nanoscale H2 sensing. (A) Schematic diagram of the sensor architecture. (B) SEM image of a Pd-nanowire array after transfer to a cyanoacrylate film. (C) Stepwise fabrication strategy. From Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 22272231. Reprinted with permission from AAAS.
’ METAL-SEMICONDUCTOR SCHOTTKY BARRIER SENSING Metal-semiconductor architectures are conceptually similar to semiconductor nanowires in exploiting the modulation of an extended SCR for chemical sensing. The SCR results from the Schottky barrier, of magnitude ϕS, that is formed when a thin metal layer is deposited on a semiconductor. When the metal and the semiconductor are brought into contact, charge flows across the interface in order to align the Fermi levels, and differences in the work functions of the metal and the semiconductor produce an interfacial electric field that penetrates both materials. The electric field can be neglected in the metal, because the electron density is large and the screening length is short. However, in nondegenerate semiconductors, band bending occurs, and an SCR is formed. The barrier height ϕS, which is the difference between the metal work function, ϕM, and the semiconductor electron affinity, χS, at the interface, correlates with the width of the SCR, with larger SCR width corresponding to smaller conductance, which can be monitored by simple impedance measurements. Chemical effects on the magnitude of the Schottky barrier, i.e., the difference between ϕS0 and ϕSH in Figure 1, form the fundamental basis for sensing at nanoscale M-S junctions. For example, H2 can adsorb and dissociate at a catalytic metal such as Pt or Pd. Free H atoms then diffuse through the metal, reach the M-S interface and form an interfacial dipole that reduces the barrier height and the width of the SCR, leading to higher conductivity in the semiconductor.23 Although direct diffusion through the metal is limited to small atomic species, clever material treatments can produce alternative sensing strategies. For example, Dobrokhotov et al. functionalized chemically inert GaN nanowires with Au nanoparticles to create an SCR in the nanowires, while still leaving room for sterically unimpeded access to the surface by small molecules. Changes in the SCR induced by adsorbed CH4 could then be measured by nanowire conductance.24 The sensitivity of M-S interfaces to gas adsorption may, among other factors, depend on the choice of the catalytic metal,
the surface state of the semiconductor, and the area of the M-S interface. Furthermore, nanoscale semiconductors are attractive, because their high surface-to-volume ratios amplify the effect of small changes in the SCR volume. Compared to a Pd-planar GaN surface, Yam et al. observed an improved sensitivity for H2 sensing when Pd was sputtered on inherently high surface area porous GaN (PGaN).25 Similar work from our laboratory showed that, by electrolessly depositing Pt inside the nanopores of PGaN, the Pt-GaN interfacial area could be increased significantly, resulting in a sensor exhibiting a detection limit of 1 ppm H2 at 300 K.26
’ METALLIC NANOWIRES Structure-Based Nanowire Sensing. Extension of nanoscale M-S structures to all-metallic nanowires is natural, inasmuch as the sensing interactions are conceptually simpler. One such class of sensors exploits analyte-induced changes in nanowire structure. Notable in this regard is the work from Penner’s laboratory on the use of Pd nanowires for gas sensing. Using the same propensity for H2 sorption that motivates the use of Pd in M-S nanosensors, they developed a strategy for hydrogen sensing that relies on changes in the structure of the Pd wire itself.27 Pd nanowires were electrodeposited from Pd(II) solution by nucleation onto step edges on graphite, viz., Figure 2. Exposure of arrays of these Pd nanowires to hydrogen in the range of 2 to 10% resulted in a reversible H2 pressure-dependent decrease in the resistance of the array. The sensor response was postulated to result from H2 sorption-induced changes in the structure of the wires caused by dilation and shrinking of Pd grains in response to hydrogen sorption and desorption, respectively. Subsequently, single Pd nanowires (70 nm < d < 300 nm) deposited using standard semiconductor processing protocols were used to detect 3
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H2 in the range of 0.02% up to 10% in N2,4 and the utility of the Pdnanowire approach was extended by fabricating the nanowires using the lithographically patterned nanowire electrodeposition (LPNE) method.28 These latter structures were prepared under conditions favoring the production of the βphase of PdHx, exhibiting mean grain diameters of ∼15 nm and consequently were able to extend the detection of H2 in N2 and air down to concentrations as low as 2 ppm. In the low [H2] regime, a dimensionally dependent sensitivity was observed, with smaller nanowires also exhibiting more rapid responses. These researchers concluded that the observed responses and recoveries were limited by hydrogen adsorption and desorption and that proton diffusion in PdHx was not limiting in either case. As Penner’s work suggests, the presence of structural or chemical alterations in the nanostructure caused by the lithographic or patterning agent can affect sensor performance, which can have significant implications for nanostructure fabrication. For example, Shi et al.29 found a stark difference between the sensitivity of nanowires prepared by electron-beam lithography (EBL) and focused ion beam (FIB) milling. Nanowires fabricated by FIB etching exhibit abnormally high resistivity and show very low sensitivity toward molecular adsorption, while those fabricated by EBL exhibit sensitive resistance changes upon being exposed to solution-phase adsorbates. Reduced sensitivity in FIB-milled nanowires was attributed to reduced grain sizes, which correlates with higher background carrier scattering and ultimately to Ga+-induced damage caused during the ion milling process. De Teresa and co-workers found a similar effect in PtC nanowires created by FIB-induced deposition.30 They observed a monotonic decrease in the nanowire resistivity with thickness, from 700 Ω cm for thicknesses g150 nm to ∼100 Ω cm for wires e20 nm, a decrease that was found spectroscopically to depend on the Pt/C ratio. Their results indicate that, in addition to beaminduced lattice damage, FIB deposition strategies can coimplant C atoms during beam-induced decomposition of organometallic precursors. To avoid some of the complications of direct-write approaches to nanowire fabrication, some groups have pursued the use of template-directed assembly of one-dimensional metallic nanowires. For example, Braun and co-workers used DNA to template the growth of 12 μm long, 100 nm wide conductive Ag wires.31 In another compelling example, Kong and co-workers utilized peptide nucleic acid probes immobilized in the gaps of microelectrodes, which were then hybridized with the cDNA analyte strands.32 Subsequently, pectin was grafted onto the DNA strand and oxidized by IO4, producing pendant aldehydes that were used to reduce ammoniacal Ag+ to metallic Ag, ultimately bridging the distance between adjacent fingers of the interdigitated microelectrode. Because the conductance increase was found to exhibit percolation behavior, the onset of the high conductance state provided a sensitive threshold sensing mechanism for [Ag+]. Boundary Layer Scattering. The other principal class of allmetallic nanowire sensors relies on electronic effects at the interface to produce boundary layer scattering. Such effects were summarized by the early phenomenological theories of Fuchs9 and Sondheimer,10 theories that were not improved upon until the early 1990s, when Persson and co-workers developed a model for diffuse scattering of carriers at surfaces which invokes the electronic structure of the adsorbate and metal in a natural way.3335 In this approach, a Newns-Anderson model for chemisorbed species is used to describe how coupling to electronhole pairs in the
substrate damps frustrated adsorbate translations, allowing the change in resistivity to be related to the adsorbate vibrational damping rate due to excitation of electronhole pairs, 1/τ. The critical parameter is the adsorbate density of states at the Fermi energy, N(EF), which is linearly related to the damping rate through, 1 2mωF Γ ¼ NðEF ÞÆsin2 θæ τ M
ð1Þ
where m = electron mass, M = adsorbate mass, Γ = width of the adsorbate density of states, and Æsin2 θæ is a geometric factor determined by the symmetry of the adsorbate molecular orbital near EF = pωF. Connection between the Persson theory and boundary layer scattering experiments occurs through N(EF), which determines the extent of adsorbate-metal electronic interaction. It predicts that increasing the adsorbate density of states at the Fermi energy increases the resistivity, F, thereby explaining observed differences in sensitivity to strongly, e.g., thiolates, and weakly, e.g., pyridine, interacting species on Au (vide infra). It is important to note the central role played by the electronic character of the adsorbate. Thus, adsorbates differing only in constituents that do not contribute to orbitals near the HOMOLUMO gap, e.g., different chain lengths but the same Au-binding moiety produce the same F. The ultimate nanowire is constrained in width to a single metal atom, the so-called string-of-pearls or atom-scale junction (ASJ). Nanostructures with atomic-scale gaps or junctions have been fabricated by a variety of methods, with mechanically controlled break junctions (MCBJ),15,36 electromigration,37,38 and electrochemical synthesis3942 being the most common choices. Tao and co-workers developed the powerful directional electrodeposition method for formation of ASJs.13 ASJs exhibit a variety of interesting behaviors, e.g., self-annealing, electromigration, spontaneous restructuring, etc., and they are much more sensitive to molecular adsorption than thicker film and wire structures (vide infra). Furthermore, the capacity to regenerate the sensor structure after a measurement means that multiple sensing cycles can be accomplished with a single nanowire. The physical mechanisms that may be used for chemical sensing at metallic nanowires are typically developed directly from the special characteristics of the wires themselves. Because metallic nanowires exhibit enormous surface-to-bulk atom ratios compared to larger structures, boundary layer scattering is dominated by electronic coupling of adsorbates to the metal, either at the surface directly or at grain boundaries. Penner’s group demonstrated the latter effect in their study of chemically reactive interfacial boundaries. They found that Ag mesowires fabricated by electrochemical step-edge deposition show differential resistance behavior upon exposure to different adsorbates.43 For example, a reversible increase in resistance is observed upon exposure to NH3, while exposure to H2S, which is known to chemisorb irreversibly, causes an irreversible increase in the resistance of Ag mesowires, and no measurable effect was observed upon exposure to CO, hydrocarbons, or H2O. To enhance this type of effect, Liu and Searson formed AuAg alloy nanowires by electrochemical template synthesis, then etched out the Ag to form nanoporous Au nanowires.44 A 3% resistance change observed upon the self-assembly of octadecanethiol, comparable to the thick wire sensitivity limit described below, was ascribed to the increase in diffusive scattering at the surface. Atom-Scale Junctions and Sensing. ASJs represent the ultimate in ultrathin nanowires and are particularly interesting objects. The dimensions of ASJs are comparable to the Fermi wavelength of the metal, 0.52 nm for Au at 300 K, so they are 4
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Figure 3. Normalized sensitivity, ΔΖ/Ζ0, of ASJs and nanowires to adsorption of 1 mM mercaptopropanoic acid as a function of nominal wire conductance in units of G0. (Inset) Example of an assembly curve on thick wire, G > 200G0.
dominated by quantum phenomena. For example, their conductance, G, is quantized in units of G0 (G0 = 2e2/h ∼ 77.5 μS), with typical values for ASJs in the range of 110 G0, and thicker ASJs exhibit higher G values. These properties allow the formation of ASJs to be monitored in situ by simple electrical measurements. Once formed, the goal is to follow adsorption at the ASJ through changes in ballistic electron transport, so the mechanism by which adsorbates alter ballistic electron transport through ASJs has been studied both theoretically and experimentally. Schmickler and co-workers used density functional theory to investigate the electrochemical reactivity of Au, Cu, and Ag monatomic nanowires to hydrogen.45 H atom adsorption was found to be much stronger on Cu and Au wires than on Ag. These results were then used to explain observations of fractional quantum conductance (i.e., G = G0/n, n an integer) on Cu and Au nanowires in the hydrogen evolution region of applied potential. Parallel experimental work was accomplished in our laboratory using directional electrodeposition between Au electrodes, accomplished by applying a potential between the thin film electrodes, to etch Au from the anode and deposit it on the cathode, thereby closing the gap. Current through the gap is monitored continuously, and the directional electrodeposition is terminated when a current near that corresponding to the conductance quantum, G0 = 2e2/h, is reached. Then, the ac impedance is measured while hexadecanethiol (HDT) chemisorbs onto the ASJ. Boundary layer scattering from chemisorbed HDT was found to produce a normalized impedance change of 71 ( 1%, the noise level being equivalent to a population fluctuation of (5 HDT molecules.46 As shown in Figure 3, the sensitivity of ASJs is maximized for junctions exhibiting conductance below ∼20G0. The principal problem in achieving such junctions is metal overgrowth. To overcome this problem, we identified a unique open working
Figure 4. ASJ fabrication strategy. (Top) SEM of the (50 nm typical) initial nanogap, inset shows an ∼8 nm gap. (Bottom) Schematic diagram of the open-WE approach to slow, controllable growth of ASJs.
electrode (open-WE) strategy to slow down the rate of electrodeposition.47 Starting from a SiNx-covered, photolithographically defined Au microbridge (∼3 μm wide, 100 μm long), a nanogap (5 nm < d < 100 nm) is opened in the Au microbridge using FIB milling, after which metal ASJs are regrown electrochemically. To make atomic scale contacts, fine control over the contact size is 5
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production of AuAgAu and AuCuAu bimetallic ASJs exhibiting remarkable stability, e.g., steady conductance values for over 3 h in the best AuAgAu junctions. AuCuAu ASJs display more complicated behavior, highlighted by spontaneous restructuring dynamics at open circuit. These dynamics, if unchecked, always result in a G = 0 state, i.e., a broken wire, after a few minutes. However, if a sufficiently negative potential, EDC, is applied, the AuCuAu structures can be stabilized at small conductance values, as illustrated in Figure 5. The stable structures exist for periods of up to tens of minutes; over which time, the slowed restructuring dynamics can produce a number of intermediate states, even fractional conductance states at long times. The development of a stable fractional conductance state for both Cuand Ag-based bimetallic ASJs likely has its origin in physical restructuring. Thus, reproducible fabrication of bimetallic ASJs can be achieved by choosing metals such that the nanogap substrate (Au) is inert in the electroplating solution under the applied potentials needed to deposit or dissolve the second metal, the resulting AuAgAu and AuCuAu bimetallic ASJs being stable both on open Si surfaces and in microfluidic channels. This capability opens the door to exciting applications of bimetallic ASJs, for example, as renewable detection elements in microfluidic devices for chemical and biochemical sensing. Other methods of fabricating ASJs are also amenable to the production of chemical sensors. For example, Kiguchi and coworkers developed an electrochemical version of the MCBJ to measure the electric conductance of metal nanowires under electrochemical potential control.49 The conductance of a Au ASJ in aqueous Na2SO4 showed clear peaks in the conductance histogram at integral values of G0 and maintained stable atomic contacts for >5 s. Hepel explored the formation of Ni ASJs and their dependence on longitudinal electric field through fieldinduced formation and field stabilization.50 Ni nanowires were found to be stable only in the presence of an electric field, rapidly dissociating when no field was present. Stable monatomic ASJs were obtained in electrolyte solutions, which exhibited quantum conductance at low electric fields, switching to thermionic conductance at higher field. Although these approaches have not, as yet, produced ASJs with the temporal stability of those obtained by electrochemistry, the structural control afforded by MCBJs and the promise of new materials offers ample motivation for continued research into these fabrication schemes.
Figure 5. Conductance histograms of AuCuAu bimetallic ASJs under (top) open circuit and (bottom) EDC = 1.3 V (WE open). Inset: Example of spontaneous restructuring in a AuCuAu bimetallic ASJ.
critical. Unfortunately, this is difficult to achieve using standard electrodeposition. Ideally, the deposition (dissolution) proceeds at an extremely slow rate, so that the fabrication can be terminated without overgrowing (dissolving) the junction. To accomplish this, an open-WE was developed, in which a potentiostat, providing a DC potential EDC, is connected to a Au wire quasi-counter/reference electrode (QCRE), while the nanogap electrode pair is kept at open circuit potential. Electrodeposition or electrodissolution (depending on the value of EDC) can proceed under these openWE conditions, but at extremely slow rates, so that the junctions can be terminated at any desired conductance value by simply disconnecting the QCRE. Figure 4 illustrates the fabrication of the FIBmilled Au nanogap starting structures and the open-WE deposition strategy. Coating the WE with a protective Si3N4 layer is critical for the subsequent electrochemical fabrication, because it minimizes leakage current arising from ionic conduction of the electrolyte and also protects most of the exposed Au surface, so that electrodeposition is confined to the nanogap area. The ASJs fabricated in this way display a variety of behaviors characteristic of quantum systems, including spontaneous restructuring, resulting in step changes in the conductance between well-defined multiples of G0. Bimetallic ASJs. The open-WE method was employed to successfully fabricate robust Au ASJs (stable >1 h, in favorable cases) in FIB-milled Au nanogaps. However, repeated electrodissolution of overgrown junctions results in irreparable damage to the Au nanogaps because of the poor site-selectivity of electrodissolution. Since the ultimate goal is to integrate ASJs into microfluidicnanofluidic architectures as renewable chemical sensing elements, the ability to regenerate functional ASJs is critical. To circumvent the electrodissolution problem, bimetallic junctions were fabricated as AuXAu structures, with X denoting a metal more easily oxidized than Au.48 Using the open-WE protocol, extremely slow electrochemical reaction rates were achieved, greatly enhancing the controllability of nanofabrication and allowing
’ CONCLUSIONS Conductance-based chemical sensing in metallic and metalsemiconductor (M-S) nanostructures can be mediated either by analyte-induced structural changes or through direct electronic interactions. The nanoscale nature of these structures is important in two ways. First, the enhanced surface-to-volume ratio which accompanies a decrease in device dimensions produces structures in which a larger portion of the structure is active and contributes to chemical sensing. This can lead to improved sensitivity, especially in the case of M-S structures, in which sensing is mediated through modulation of the magnitude of the SCR. In addition, when metallic structures approach atomic dimensions, conductance is mediated by ballistic transport, which exhibits enhanced sensitivity to molecular adsorption relative to larger structures. Finally, the inclusion of metals also enhances the sensing capabilities of these structures by bringing the catalytic properties of the metals into play, as has been extensively used in gas sensors employing either Pt or Pd. 6
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The principal challenges in increasing the use of these nanostructures lie in gaining better control over their production and temporal stability. However, given the improvements that have recently been realized through electrochemical fabrication of ASJs and the production of M-S structures with catalytic metals for high temperature applications, we can anticipate increasing penetration of these structures into field-deployed sensor architectures.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ BIOGRAPHY Barrett K. Duan received the B.S. in chemistry from the University of Wisconsin-Madison and is a Ph.D. candidate in chemistry working under the supervision of Prof. Paul Bohn at the University of Notre Dame. His current research focuses on developing highly sensitive and stable chemical sensors based on nanoscale semiconductor materials. In addition, he studies the chemical interactions between analytes and metal-semiconductor sensors. Jingying Zhang received the B.S. in Materials Science & Engineering from the University of Science and Technology of China and is a Ph.D. candidate in chemical and biomolecular engineering working under the supervision of Prof. Paul Bohn at the University of Notre Dame. Her research interests are focused on fabrication and the electrical and chemical sensing properties of metallic nanowires. Paul W. Bohn received the B.S. in Chemistry from the University of Notre Dame du Lac in 1977 and the Ph.D. in Chemistry from the University of Wisconsin-Madison in 1981. After a two-year stint at Bell Laboratories, he joined the faculty at the University of Illinois at UrbanaChampaign (UIUC), where he remained until moving to Notre Dame in 2006. He is currently Arthur J. Schmitt Professor of Chemical and Biomolecular Engineering, Professor of Chemistry and Biochemistry, and Director of the Advanced Diagnostics and Therapeutics Initiative. His research interests include: integrated nanofluidic and microfluidic chemical measurement strategies for personal monitoring, chemical and biochemical sensing in mass-limited samples, smart materials, and molecular approaches to nanotechnology. ’ ACKNOWLEDGMENT Work described here that was conducted in the authors’ laboratories was supported by the National Science Foundation (NSF0807816) and by the Army Corps of Engineers (contract W9132T-07-2-0003). ’ REFERENCES (1) Baresel, D.; Gellert, W.; Sarholz, W.; Scharner, P. Sens. Actuators 1984, 6, 35–50. (2) Windischmann, H.; Mark, P. J. Electrochem. Soc. 1979, 126, 627–633. (3) Wanekaya, A. K.; Chen, W.; Myung, N. V.; Mulchandani, A. Electroanalysis 2006, 18, 533–550. (4) Yun, M. H.; Myung, N. V.; Vasquez, R. P.; Lee, C. S.; Menke, E.; Penner, R. M. Nano Lett. 2004, 4, 419–422. (5) Cui, Y.; Wei, Q. Q.; Park, H. K.; Lieber, C. M. Science 2001, 293, 1289–1292. (6) Law, M.; Goldberger, J.; Yang, P. D. Ann. Rev. Mater. Res. 2004, 34, 83–122. 7
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