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U. Chandni , * Paromita Kundu , Subhajit Kundu , N. Ravishankar , and Arindam Ghosh

Tunability of Electronic States in Ultrathin Gold Nanowires

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Shrinking metallic systems to molecular scale makes the effects of the surrounding environment and the charge confi gura-tions extremely important to their electrical properties. This is particularly true for one-dimensional (1D) or quasi-1D metals if these are to be used as reliable interconnects or electronic circuit elements. In 1D, Coulombic interaction and random background disorder form the major factors that determine the charge transport. [ 1–3 ] Recent experiments demonstrate that the Coulombic interaction may lead to new quantum many-body phases, for example the Tomonaga-Luttinger liquid (TLL) [ 4–6 ] in atomically thin gold nanowires, [ 7 , 8 ] but both the nature and effect of disorder on charge transport in these phases remain virtually unknown.

In low dimensional systems, disorder can have contributions from both extrinsic (such as a supporting substrate or charge environment) and intrinsic (such as lattice defects or structural corrugations) factors, that in turn strongly modify the effects of Coulomb interaction, and becomes instrumental in deter-mining the ground state of the system. Experimental results for 1D/quasi-1D systems are primarily dichotomous: (i) signa-tures of Tomonaga-Luttinger liquid (TLL), an exotic many-body state caused by the Coulombic interaction, in a wide variety of materials including carbon nanotubes, [ 4 , 6 ] semiconductor [ 5 ] and metallic nanowires, [ 7 ] inorganic/organic nanowires [ 9 ] to polymer nanofi bres [ 10 ] and (ii) variable range hopping (VRH), which usually implies strong localization of carriers. [ 11–14 ] It is real-ized that disorder plays a crucial role in this dichotomy, but in spite of attempts involving mechanically suspended devices, [ 15 ] alternative dielectric environment, [ 16 ] or thermal treatments, [ 17 ] a controlled mechanism of separating the infl uence of extrinsic from intrinsic disorder on carrier dynamics in 1D systems con-tinues to remain an experimental challenge.

The present work addresses this issue by using ultrathin metal (Au) nanowires grown on SiO 2 /Si and Si 3 N 4 /Si substrates which were separated from the substrate via organic under-layers or linkers of varying thickness. We have grown single-crystalline Au nanowires of 2 nm diameter by oriented attach-ment of Au nanoparticles initiated by the selective removal of oleylamine capping agent from the (111) facet of the cubocta-hedral particles. The nanowires were directly grown on various substrates and subsequently contacted with specially designed

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U. Chandni, A. GhoshDepartment of PhysicsIndian Institute of Science, Bangalore 560 012, India E-mail: chandni@physics.iisc.ernet.in P. Kundu, S. Kundu, N. RavishankarMaterials Research CentreIndian Institute of ScienceBangalore 560 012, India

DOI: 10.1002/adma.201204493

Adv. Mater. 2013, DOI: 10.1002/adma.201204493

electron beam lithography for electrical transport measure-ments. The details of the growth and contacting method can be found in Refs. 7, 18, 19. Here, we have demonstrated a novel technique by which the separation between the nanowires and the substrate could be varied, which dramatically changes their transport properties.

We exploit the amine-SiO 2 affi nity, amine-ascorbic acid chemistry (details in supplementary information) and the dif-ference in interaction energy between Au/SiO 2 and Au/Si 3 N 4 substrates as mediated by an organic interlayer to synthesize wires located at varying spacings from the underlying substrate. The van der Waals interaction energy between Au and SiO 2 or Si 3 N 4 via oleyl amine as the medium is estimated (assuming a confi guration of cylinder lying on a planar surface) from the calculation of the Hamaker constant (A) [ 20 ] (details in the sup-plementary information). The interaction is attractive for Au/SiO 2 (positive value of A) while it is repulsive in the case of Au/Si 3 N 4 . Thus, a cleaning step after synthesis leads to a removal of the interlayer in the case of Au/SiO 2 while the interlayer is not removed in the case of the Au/Si 3 N 4 . This presents us two cases in which the wire is directly in contact with the substrate and one in which it is separated by ∼ 2 nm from the substrate.

A further modifi cation is possible in the case of Au/SiO 2 which leads to the formation of a much thicker ( ∼ 6 nm) inter-layer between the wire and the substrate. While in the ear-lier scheme for the growth of wires on SiO 2 , an intermediate cleaning step was employed, in this modifi ed scheme of growth, this intermediate cleaning step was not used. The oleylamine bilayer (OAm) adsorbed on SiO 2 substrate and the capping layer attached to the Au nanoparticles that participate in the nanowire growth (OAm’) are held together by an ascorbic acid bridge, denoted as AA bridge in Figure 1 a. This leads to the formation of the ∼ 6 nm thick interlayer between the substrate and the wire. In the earlier case, the amine layer attached to the substrate (OAm) is cleaned off during the intermediate cleaning step thus resulting in the formation of a single layer of oleylamine (OAm’) between the substrate and the wire. However, in the present case, the absence of this intermediate cleaning step leads to the formation of a thicker bilayer of amine. In the fi nal cleaning step, the large spacing between the wire and the substrate considerably weakens the attractive van der Waals interaction between the Au wire and the SiO 2 sub-strate and thus this interlayer is not removed by this cleaning process.

The above mentioned reaction schemes, thus provide us three sets of unique samples primarily differing in the distance of the wire from the substrate. In the devices, referred as d 6, Au nanowires were grown on the SiO 2 substrate [ 21 ] without any intermediate cleaning step. In the second set, denoted as d 2, the wires were grown on Si 3 N 4 substrates, which have low affi nity to oleylamine. Consequently, only one oleylamine bilayer (OAm’) of ≈ 2 nm separates the nanowires from the

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Figure 1 . Architectures of devices d 6, d 2 and d 0 where the wire is placed at a height of ≈ 6 nm, ≈ 2 nm from and on the substrate (i. e., height = 0), respectively. In (a), d 6, the organic underlayer is formed by bridging the amine capping on wire (OAm’) and the amine (OAm) adsorbed on SiO 2 with ascorbic acid (AA); (b) d 2 where the organic spacer between Au and Si 3 N 4 substrate is only the OAm’ and there is no layer between wire and SiO 2 substrate in (c), d 0. AFM topographic images for device architectures (d) d 6, (e) d 2 and (f) d 0. The corresponding height profi les are shown in panels (g)-(i). (j) Collated histogram of the height profi les taken from over fi ve devices each, for the three nanowire architectures, confi rming the presence of an underlayer of thickness d ≈ 5.5 − 6 nm and d ≈ 2 − 2.2 nm for d 6 and d 2 respectively. Clearly, d 0 consists of the bare wire with a height profi le of ≈ 2 nm.

substrate (Figure 1 b). Thus, in d 6 and d 2, the presence of an underlayer enables separation of the wires from the substrate-induced extrinsic disorder sites, an ideal situation to study the effects of intrinsic disorder on the transport properties. Finally, in the third set of devices ( d 0), the wires are directly in contact with the bare SiO 2 substrate (Figure 1 c) enabled by cleaning

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of the organic layers prior to growth. This provides a plat-form to evaluate the effects of possible charge traps that act as extrinsic disorder sites. Atomic force microscopy (AFM) studies (Figure 1 d-f) confi rm the height of the wires on the substrate to be ≈ 7–8 nm, ≈ 4 nm and ≈ 1.9 nm respectively, indicating the presence of an underlayer or an organic linker in device

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Figure 2 . (a) Device structure for the wires showing an oleylamine underlayer of varying thickness d . The device consists of either bunches of nanow-ires stacked in a horizontal direction or one/two nanowires. Transport measurements on d 6 with a bunch of nanowires showing (b) current-voltage ( I − V ) characteristics that turn highly non-linear at low T . (c) Normalised linear bias R vs. T for device architectures d 6 (open symbols) and d 2 (closed symbols), exhibiting power law behavior. The exponents were found to decrease with increasing number of wires. [ 7 ] Circles indicate one/two wires ( α ≈ 3.5 − 6), while diamonds indicate bunches of wires ( α ≈ 1 − 2). Scaling of I − V s for device architectures (d) d 6 and (e) d 2. The red line indicates the fi t to equation 1 in the text. In case of d 6, the scaling was done from 300 K–10 K, for a bunch of nanowires, giving α = β = 1.56 and γ = 0.25. For d 2, scaling was done for a single nanowire device in the temperature range 270 K–80 K, giving α ≈ 2 β = 6.2 and γ = 0.65

architectures d 6 and d 2, whereas d 0 consists of the bare wire on the substrate. This is validated using more than fi ve samples and multiple line scans (sample scans are given in Figure 1 g-i), a histogram of which is plotted in Figure 1 j.

For electrical measurements, OAm was removed from the upper surface of the nanowires before depositing Au or Ti/Au contact pads. This led to low contact resistance as verifi ed with four probe devices. [ 7 ] The fragility of the ultra-thin nanowires forced us to develop a special low-temperature electron beam lithography process which is outlined in Ref. 7. Even at room temperature, both two and four probe resistance of the nanowires varied from 0.05 − 25 M Ω irrespective of the device architecture or device dimensions, corresponding to resistivity that is about 2-1000 times larger than the bulk crystal. The single crystallinity, notwithstanding isolated lattice defects, of the nanowires was confi rmed with Transmission Electron Microscopy (see supplementary information), indicating an unexpected transport regime.

We fi rst focus on the d 6 and d 2 architectures because they are expected to show very similar transport characteristics due to the presence of the organic underlayer. Figure 2 b shows the current-voltage ( I − V ) characteristics obtained for a bunch of wires belonging to d 6 which turned strongly non-linear at lower T , indicating insulating nature of the wires. This refl ects in the temperature-dependence of linear resistance R , obtained from

© 2013 WILEY-VCH Verlag GmAdv. Mater. 2013, DOI: 10.1002/adma.201204493

the slope of the I − V traces at V ≈ 0, which was found to follow a power law with R ∼ T − α . Similar characteristics were obtained for all devices belonging to d 2 and d 6 architectures, irrespec-tive of individual or bunched confi gurations, except the slope α was found to be lower in the bunched devices. In 1D, power-law transport is often attributed to the manifestation of TLL ground state. Recent experiments claim this possibility in both ultra-thin [ 7 ] and atomic scale. [ 8 ] Au nanowires, where the power law variation in R − T arises from a TLL-TLL tunneling from the bulk of the wire, separated by one (or few) dominant barrier(s), that acts as a strong back scatterer(s). To verify this further, the I − V characteristics of the d 6 and d 2 devices were scaled in accordance with the generic TLL form [ 5 , 22 ]

I(V, T ) = I0Tα+1 sinh

(γ eV

2kB T

) ∣∣∣∣�(

1 + β

2+ i

γ eV

2πkB T

)∣∣∣∣2

(1)

where I 0 is a constant, γ signifi es the number of tunnel barriers where the applied voltage gets dropped, Γ is the gamma func-tion, and α and β are exponents that depend on the strength of Coulomb interaction. As shown in Figures 2 d and 2 e, we fi nd excellent scaling of I − V s for all the devices measured. The value of γ was found to be ≈ 1 in about 60% of the devices and α ≈ β in > 80% cases. Both power law R − T and the generic scaling of I − V provide strong evidence that low energy excitations in

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Figure 3 . Scanning electron micrographs for various device confi gurations showing (a) single, (b) network and (c) bunch of nanowires. Inset to (c) shows transmission electron micrograph of a bunch of nanowires, with inter-wire separation roughly equal to the nanowire diameter. (d) Linear bias R vs. T for various confi gurations for device architecture d 0, exhibiting a R ∼ exp [( T 0 / T ) 1/2 ] dependence for the various confi gurations. (e) R showed a clear deviation from a power-law behavior for all the device confi gurations. The curves have been offset for clarity. (f) The variation of the average value of the correlation energy T 0 for the different device confi gurations, indicating a lower T 0 for bunch/network as compared to one/two-wire(s) devices.

the nanowires in d 6 and d 2 architectures correspond to a TLL. (In addition, we have also tuned the single particles states in confi ned segments of the nanowires with an external gate bias, which yields resonant tunneling behavior pertaining to TLL-like ground state, [ 23 ] see supplementary material).

In contrast, devices from d 0 architecture showed a very dif-ferent nature of transport. Here, transport measurements were carried out in three nanowire confi gurations, i.e., isolated indi-vidual nanowires ( Figure 3 a), non-parallel nanowires forming a multiply connected extended network (Figure 3 b) and bunched nanowires with multiple parallel wires forming ribbon like structures (Figure 3 c). Although the I − V characteristics were nonlinear, we found that the zero bias resistance R deviated con-siderably from the power law behavior in all cases (Figure 3 e). Instead, R was found to follow a VRH-like behavior given by, [ 24 , 25 ]

R = R0 exp [(T0/T )ν ] (2)

where R 0 and T 0 are the resistance prefactor and correlation energy respectively, and ν is the VRH exponent. As shown in Figure 3 d, we fi nd the data to be best fi tted with ν = 1/2 in all the device confi gurations within d 0 architecture. (See Supple-mentary Material for fi ts to ν = 1/3, 1/4 and 1 for comparison.) More than ten devices in these confi gurations were tested, and all showed clear VRH-type transport from room temperature down to 4 K.

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In 1D, VRH with ν = 1/2 can arise from multiple processes. A non-interacting Mott VRH ( R ∼ exp[(T0/T )

1d+1 ] ), describing

transport in a strongly disordered system with localized states, naturally leads to a ν = 1/2 dependence in 1D. [ 24 ] In our case, a localized state of the wires from strong structural distor-tions due to surface roughness of the SiO 2 substrate could be envisaged, but unlikely because AFM studies measures surface roughness ( ∼ 0.2 nm) to be only about 10% of the wire dia meter. Alternatively, the long range Coulomb interaction between localized electrons, leads to Efros-Shklovskii (ES) type VRH, due to a soft Coulomb gap in the density of states at Fermi energy. [ 26 ] In addition, a disordered TLL in localized phase [ 27 ] as well as pinned charge density waves in the presence of strong impurities [ 28 ] are expected to display VRH-like transport with ν = 1/2. In the latter, strong pinning centers disintegrate the Wigner Crystalline order into smaller locally ordered segments, and at low T , transport shows a VRH-like T − dependence due to hopping of electrons between these segments or rods. Indeed, the role of interaction is evident from the dependence of the correlation energy T 0 on the device confi guration. As shown in Figure 3 f, T 0 ( ≈ 2500 K) for individual nanowires is ∼ 20 and ∼ 30 times larger than that for bunches and networks. This is a result of enhanced screening of Coulomb potential in an ensemble of wires that can cause net T 0 to decrease in different ways, for example, weakening of inter-wire interaction, [ 29 ] or

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de-stabilizing the pinned Wigner crystalline phase, and so on. Nevertheless, the most important implication of VRH transport shown in Figure 3 is the fact that charge carriers in d 0 architec-ture are localized, in stark contrast to d 2 and d 6, where a linker separates the nanowires from the substrate.

The above observations lead to the key question of this work: Why proximity to the substrate localizes the charge carrier in a Au nanowire. The fact that the TLL behavior is restored in archi-tectures that contain linkers of fi nite thickness, and that the transport characteristics remain essentially unchanged for linker thicknesses of 2 nm ( d 2) and 6 nm ( d 6), it can be concluded that dominant disorder in these cases is intrinsic in origin. Stacking faults, twin boundaries or surface modulation due to intrinsic Rayleigh instability [ 30 ] can act as strong back scattering sites along the length of the nanowire, causing disintegration of the TLL, and transport occurs via TLL-TLL tunneling (Supplemen-tary Figure. S10 depicts this scenario). Hence localization of car-riers in the d 0 architecture is driven by extrinsic disorder induced by the substrate, because intrinsic disorder alone is not enough to drive the system to localization. In view of the small surface roughness, the most likely origin of this is the Coulomb potential arising from the trapped charges at the SiO 2 substrate. The pres-ence of a linker layer can lead to distinct effects: (1) The func-tional groups in the linker which are electronegative can lower the electron number density of the nanowire (2) The presence of an additional layer can lead to enhanced screening from the charge traps in the substrate, as the linker now acts as a dielec-tric (3) The linker here increases the separation of the nanowire from the charge traps and hence leads to a lowered interaction. A simple order-of-magnitude estimation of the localization length ξ supports this hypothesis which is discussed below and schemati-cally demonstrated in Supplementary fi gure S10.

We focus on the individual nanowire devices for which T 0 ≈ 2500 K. In spite of several underlying mechanisms of VRH ( ν = 1/2) transport in 1D, the relation between T 0 and ξ is generic, i.e., T 0 = C / ξ , where C is a model dependent proportionality factor. For a phase-pinned disordered TLL, C = ( 4π

g )2 2�vFkB

, where g and v F are the interaction parameter and Fermi velocity respectively, [ 27 ] which yields unphysically large ξ ( > 20 μ m). In the Efros-Shklovskii models [ 26 ] , as well as the disintegrated charge density wave scenario, [ 28 ] C = β e 2 /4 π ∈ k B , where β ∼ 1, which gives ξ ∼ 5 nm. If localization is caused by the fl uctuations in the conduction band caused by the charge trapped at the wire-substrate interface, one can estimate ξ from ξ ∼ �/

√(2mφC ) ,

where ϕ C is the Coulomb potential from the trapped charge. Bare Coulomb potential where ϕ C = e 2 /4 π ε 0 r , where r = 2 nm, gives ξ ≈ 1.5 nm, while assuming ϕ C as screened Coulomb potential provides ξ ∼ 40 nm. This implies that the external Coulomb potential from the trapped charges remain nearly unscreened, a striking result considering the metallicity of Au.

Interestingly, physisorption on to a surface, is a distinct mechanism that is known to create disorder sites in many nano-scale systems including graphene [ 1 ] and carbon nanotubes, [ 2 ] due to residual interactions with the substrate. This can have very strong effects on the geometric structure which have been studied using AFM, [ 31 ] scanning tunneling microscopy [ 3 ] and molecular-mechanics simulations. [ 32 ] Structures on substrate are expected to have strong elastic scattering sites in the form of large barriers often leading to localization. [ 2 ]

© 2013 WILEY-VCH Verlag GAdv. Mater. 2013, DOI: 10.1002/adma.201204493

To conclude, by using a carefully structured linker layer that chemically adheres the nanowire to the substrate, we have obtained clear control on the infl uence of extrinsic and intrinsic disorder sites on the transport. The method discussed here could very well be adapted to other metal-semiconductor 1D structures as well. We obtain a clear crossover from a non-Fermi liquid TLL ground state to a disordered state with VRH mediated transport. The TLL is established via clear power-laws, whereas a VRH governed by electron-electron interactions defi ne the localized regime. The study provides a novel method to tune the transport in metal wires on substrates, which could be relevant to their growing needs as interconnects or circuit elements.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements A.G. and N.R. acknowledge funding from the Department of Science and Technology. U.C. thanks Council of Scientifi c and Industrial Research for a fellowship.

Received: October 30, 2012 Revised: January 27, 2013

Published online:

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