Carbon nanotube transistors have outstanding potential for electronic detection of biomolecules in solution. The physical mechanism underlying sensing however remains controversial, which hampers full exploitation of these promising nanosensors. Previously suggested mechanisms are electrostatic gating, changes in gate coupling, carrier mobility changes, and Schottky barrier effects. We argue that each mechanism has its characteristic effect on the liquid gate potential dependence of the device conductance. By studying both the electron and hole conduction, the sensing mechanisms can be unambiguously identified. From extensive protein-adsorption experiments on such devices, we find that electrostatic gating and Schottky barrier effects are the two relevant mechanisms, with electrostatic gating being most reproducible. If the contact region is passivated, sensing is shown to be dominated by electrostatic gating, which demonstrates that the sensitive part of a nanotube transistor is not limited to the contact region, as previously suggested. Such a layout provides a reliable platform for biosensing with nanotubes.

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...

We report reproducible fabrication of InP-InAsP nanowire light emitting diodes in which electron-hole recombination is restricted to a quantum-dot-sized InAsP section. The nanowire geometry naturally self-aligns the quantum dot with the n-InP and p-InP ends of the wire, making these devices promising candidates for electrically-driven quantum optics experiments. We have investigated the operation of these nano-LEDs with a consistent series of experiments at room temperature and at 10 K, demonstrating the potential of this system for single photon applications.Nanowire light emitting diodes (NW LEDs) offer exciting new possibilities for opto-electronic devices. Growth of direct-bandgap NWs on Si 1, 2 will allow optically active elements to be integrated with already highly mature Si technology. For solid-state lighting applications, broad-area LEDs made from NW arrays have higher light-extraction efficiency than traditional planar LEDs 3 , and in the field of quantum optics, NWs offer the possibility to control electron transport at the single-electron level 4 and light emission at the single-photon level 5 .1Since the first demonstration of GaAs NW LEDs in 1992 6 , different geometries and materials have been used to produce NW LEDs operating over a wide range of wavelengths 3, 7-10 . Single-NW LEDs with doping modulation in the axial direction, which is the most interesting geometry for many applications, have been fabricated using GaN-GaInN multi-junctions 3 and a proof-of-principle device has been shown using InP 7 . In this letter we describe the fabrication and characterization of reproducible axial InP NW LED devices, and show that an active InAsP quantum dot region can be incorporated into these devices. The axial geometry allows for controllable injection of electrons and holes into the precisely defined active region, with the additional advantage of high light-extraction efficiency since the optically active region is not embedded in a high refractive index material. Unlike GaInN, InAsP emission can be tuned to infra-red telecommunications wavelengths where there is strong interest in electrically driven single-photon sources 11 .Nanowire p-n junctions were reproducibly grown in the vapor-liquid-solid (VLS) growth mode 12 by use of low-pressure metal-organic vapour-phase epitaxy (MOVPE). 20 nm colloidal Au particles were dispersed on (111)B InP substrates, after which the samples were transferred to a MOVPE system (Aixtron 200), and placed on a RF-heated gas foil rotated graphite disc on a graphite susceptor. The samples were heated to a growth temperature of 420 °C under phosphine (PH 3 ) containing ambient at molar fraction χ PH3 = 8.3×10 -3 , using hydrogen as carrier gas (6 l/min H 2 at 50 mbar). After a 30 s temperature stabilization step, the NW growth was initiated by introducing trimethyl-indium (TMI) into the reactor cell at a molar fraction of χ TMI = 2.2×10 -5 . During the first 20 minutes, hydrogen sulfide (χ H2S = 1.7×10 -6 ) was used for n-type doping, after which the p-type NW part was g...

The electronic structure of a NT is elegantly described by the quantization of wave states around a graphene cylinder 1 . Graphene is a zero band-gap semiconductor in which the valence and conduction states meet at two points in k-space, K 1 and K 2 (Fig. 1a). The dispersion around each of these points is a cone (Fig 1b). When graphene is wrapped into a cylinder the electron wave number perpendicular to the NT axis, , is quantized, satisfying the boundary condition πD = 2πj where D is the NT diameter and j is an integer. The resulting allowed k's correspond to the horizontal lines in

Carbon nanotube transistors show tremendous potential for electronic detection of biomolecules in solution. However, the nature and magnitude of the sensing signal upon molecular adsorption have so far remained controversial. Here, the authors show that the choice of the reference electrode is critical and resolves much of the previous controversy. The authors eliminate artifacts related to the reference electrode by using a well-defined reference electrode to accurately control the solution potential. Upon addition of bovine serum albumin proteins, the authors measure a transistor threshold shift of −15 mV which can be unambiguously attributed to the adsorption of biomolecules in the vicinity of the nanotube. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2775090͔Biosensors based on nanoscale field-effect transistors have the potential to significantly impact drug discovery, disease screening, biohazard screening, and fundamental science.1 The adsorption of biomolecules on the sidewall of a semiconducting carbon nanotube ͑CNT͒ or nanowire causes changes in local electrostatic environment, thereby changing the conductance of the nanomaterial. Pioneering work has indicated that this modulation of conductivity can be utilized to build CNT-based 2-11 and nanowire-based 1,12 sensors for real-time electrical detection of proteins or DNA. These sensors must be carefully designed to give reliable measurements of biomolecule binding. Of critical importance is the electrostatic potential of the solution which strongly affects the conductivity of the nanomaterial. The solution potential was not well controlled in many CNT biomolecule-binding experiments.3-10 Reported conductance changes can have little or no relationship to interactions between biomolecules and the CNT transistor, leading to unreliable sensors and hindering efforts to determine sensing mechanisms. We show that a major biosensing artifact can be removed by using a well-defined reference electrode to accurately control the solution potential.Carbon nanotube biosensors are generally constructed as shown in Fig. 1͑a͒. [3][4][5][6][7][8][9][10] The device is exposed to solution, allowing protein adsorption on the semiconducting CNT. A metal wire is used to control the electrostatic potential of the solution. A gate voltage V g , applied to the metal wire, can tune the conductance of the CNT, while a small bias eV bias Ͻ k B T is used to monitor the CNT conductance ͓for example, see Fig. 1͑b͒, curve 1͔. The electrostatic potential difference between the solution and the CNT is determined by the applied gate voltage V g and the interface potential at the metal-liquid interface V interface . This interface potential depends sensitively on electrochemical reactions occurring at the metal-liquid interface. Larrimore et al. have recently tested CNT sensors in solutions where this electrochemistry was controlled using a high concentration of a potentialdetermining redox couple.13 Sensors for biomolecule binding are generally operated in buffer solutions where ...