Tuning Carbon Nanotube Band Gaps with Strain

Abstract

We show that the band structure of a carbon nanotube (NT) can be dramatically altered by mechanical strain. We employ an atomic force microscope tip to simultaneously vary the NT strain and to electrostatically gate the tube. We show that strain can open a bandgap in a metallic NT and modify the bandgap in a semiconducting NT. Theoretical work predicts that bandgap changes can range between ± 100 meV per 1% stretch, depending on NT chirality, and our measurements are consistent with this predicted range. PACS numbers: 62.25.+g, 71.20.Tx, 73.63.Fg, 81.07.De, 85.35.Kt The electronic and mechanical properties of carbon NTs make them interesting for both technological applications and basic science. A NT can be either metallic or semiconducting depending on the orientation between the atomic lattice and the tube axis [1,2]. NTs can accommodate very large mechanical strains [3] and have an extremely high Young's modulus [4]. Both theory and experiment indicate that NTs also have interesting electromechanical properties [5][6][7][8][9][10][11][12]. A pioneering experiment [10] showed that the conductance of a metallic NT could decrease by orders of magnitude when strained by an atomic force microscope (AFM) tip. The authors suggest that a local distortion of the sp 2 bonding where the NT is touched by the AFM tip causes the drop in conductance. In Ref.[12], however, it is argued that the observed drop in conductance is due to a bandgap induced in the NT as it is axially stretched [5,8,11] as illustrated in Fig. 1(a). Evidence for the effect of strain on NT bandgap also comes from recent STM work on semiconducting NTs containing encapsulated metallofullerenes [13]. The authors found a bandgap reduction of 60% at the expected positions of the metallofullerenes and postulated that strain could account for this change.Here we present measurements to demonstrate conclusively that strain modulates the band structure of NTs. We employ an AFM tip to simultaneously vary the NT strain and to electrostatically gate the tube. We find that, under strain, the conductance of the NT can increase or decrease, depending on the tube. By using the tip as a gate, we show that this is related to the increase or decrease in the bandgap of a NT under strain. The magnitude of the effect and its dependence on strain are consistent with theoretical expectations.The samples consist of NTs suspended over a trench and clamped at both ends by electrical contacts [10,[14][15][16][17]. CVD growth is utilized to grow NTs with diameters between 1 and 10 nm at lithographically defined catalyst sites [18] on a Si substrate with a 500nm oxide. Metal contacts (5nm Cr and 50-80nm gold) are made using photolithography, as described previously [19]. An ashing step (400°C for 10 minutes in Ar atmosphere) removes photoresist residue and improves contact resistances. An HF etch (3 minutes in 6:1 BHF, etch rate 80 nm/min) followed by critical point drying is used to suspend the NTs [16]. Device conductances are not changed significantly by the etching/drying p...

Raman spectroscopy and imaging of graphene

“…Hence, the 17 cm 1 shift in the G band frequency of EG compared to that of bulk graphite or MCG corresponds to a biaxial stress of 2.27 GPa on EG. The strong compressive stress may affect both the physical and electronic properties of graphene, analogous to what occurs for CNTs [63,64]. Raman spectra of few layer epitaxial graphene on SiC substrates have also been reported by Faugeras et al [71] and no blueshift of the G band was observed.…”

Symmetry breaking in a mechanical resonator made from a carbon nanotube

“…Another mechanism for the I LF peak could be the piezoresistance of the nanotube, whose dependence on the displacement is quadratic to a good approximation 17 . The piezoresistance effect in nanotubes is by far strongest for positive gate voltages (where electrons tunnel from the p-doped regions of the nanotube near the metal electrodes into the n-doped region of the suspended part of the nanotube 18,19 ). However, the observed height of the peak in I LF can be as large for negative as for positive gate voltages (see Fig.…”

Schottky barriers in carbon nanotube-metal contacts

“…18,19 If only the circumferential confinement of the wave functions is considered, CNTs with chirality indices that fulfill the requirement n -m ¼ 3 p where p is an integer are metallic and those that fulfill n -m = 3 p are semiconducting. However, the curvature of the CNT wall 22 or mechanical deformations 23,24 of the CNT may induce band gaps of a few 10 s of mV also in otherwise metallic CNTs.…”