Gamini Sumanasekera

Charge Transfer Equilibria Between Diamond and an Aqueous Oxygen Electrochemical Redox Couple

Undoped, high-quality diamond is, under almost all circumstances, one of the best insulators known. However, diamond covered with chemically bound hydrogen shows a pronounced conductivity when exposed to air. This conductivity arises from positive-charge carriers (holes) and is confined to a narrow near-surface region. Although several explanations have been proposed, none has received wide acceptance, and the mechanism remains controversial. Here, we report the interactions of hydrogen-terminated, macroscopic diamonds and diamond powders with aqueous solutions of controlled pH and oxygen concentration. We show that electrons transfer between the diamond and an electrochemical reduction/oxidation couple involving oxygen. This charge transfer is responsible for the surface conductivity and also influences contact angles and zeta potentials. The effect is not confined to diamond and may play a previously unrecognized role in other disparate systems.

Hybrid Tin Oxide Nanowires as Stable and High Capacity Anodes for Li-Ion Batteries

In this report, we present a simple and generic concept involving metal nanoclusters supported on metal oxide nanowires as stable and high capacity anode materials for Li-ion batteries. Specifically, SnO(2) nanowires covered with Sn nanoclusters exhibited an exceptional capacity of >800 mAhg(-1) over hundred cycles with a low capacity fading of less than 1% per cycle. Post lithiation analyses after 100 cycles show little morphological degradation of the hybrid nanowires. The observed, enhanced stability with high capacity retention is explained with the following: (a) the spacing between Sn nanoclusters on SnO(2) nanowires allowed the volume expansion during Li alloying and dealloying; (b) high available surface area of Sn nanoclusters for Li alloying and dealloying; and (c) the presence of Sn nanoclusters on SnO(2) allowed reversible reaction between Sn and Li(2)O to produce both Sn and SnO phases.

MoO 3−x Nanowire Arrays As Stable and High-Capacity Anodes for Lithium Ion Batteries

In this study, vertical nanowire arrays of MoO(3-x) grown on metallic substrates with diameters of ~90 nm show high-capacity retention of ~630 mAhg(-1) for up to 20 cycles at 50 mAg(-1) current density. Particularly, they exhibit a capacity retention of ~500 mAhg(-1) in the voltage window of 0.7-0.1 V, much higher than the theoretical capacity of graphite. In addition, 10 nm Si-coated MoO(3-x) nanowire arrays have shown a capacity retention of ~780 mAhg(-1), indicating that hybrid materials are the next generation materials for lithium ion batteries.

Efficient hydrogen evolution in transition metal dichalcogenides via a simple one-step hydrazine reaction.

Hydrogen evolution reaction is catalysed efficiently with precious metals, such as platinum; however, transition metal dichalcogenides have recently emerged as a promising class of materials for electrocatalysis, but these materials still have low activity and durability when compared with precious metals. Here we report a simple one-step scalable approach, where MoOx/MoS2 core-shell nanowires and molybdenum disulfide sheets are exposed to dilute aqueous hydrazine at room temperature, which results in marked improvement in electrocatalytic performance. The nanowires exhibit ∼100 mV improvement in overpotential following exposure to dilute hydrazine, while also showing a 10-fold increase in current density and a significant change in Tafel slope. In situ electrical, gate-dependent measurements and spectroscopic investigations reveal that hydrazine acts as an electron dopant in molybdenum disulfide, increasing its conductivity, while also reducing the MoOx core in the core-shell nanowires, which leads to improved electrocatalytic performance.

Displacement current detection of photoconduction in carbon nanotubes

Using a capacitive photocurrent measurement technique, we demonstrate the ability of both semiconducting and metallic single wall nanotubes to function as photodetectors over a wide spectral range. We observe clear peaks in the photo induced displacement current of a nanotube-plated capacitor that correspond directly to the semiconducting and metallic transitions in the nanotube absorbance spectrum. The signal increases substantially as the carrier drift velocity is raised with applied bias. A large increase in the photocurrent observed below temperatures of 100K suggests that the nanotube hot carrier relaxation rate decreases substantially at low temperatures.

Kinetically limited de-lithiation behavior of nanoscale tin-covered tin oxide nanowires

In this paper, we report that Sn-nanocluster-covered SnO2 nanowire (“hybrid architectures”) electrodes exhibited stage-wise de-lithiation suggesting complete lithium extraction. The lithiation and de-lithiation behavior explains that the high capacity retention of 814 mAh g−1 and durability over hundred cycles is because of low irreversible capacity loss. Mono-layers of un-agglomerated, sub 60 nm size Sn clusters supported on metallic electrodes also exhibited similar stage-wise de-lithiation while the microscale Sn clusters exhibited single-phase lithium extraction. This can be attributed to shorter lithium ion diffusion lengths and high surface area of the nanomaterials. The cyclic voltammetric studies of Sn nanoclusters (sub 60 nm size) confirm the reaction kinetics limited behavior of lithiation and de-lithiation characteristics. The Sn-nanocluster-covered SnO2 nanowires showed a capacity retention of 458 mAh g−1 at 500 mAg−1 current density indicating an excellent rate capability.

Adsorption of oxygen molecules on individual single-wall carbon nanotubes

Our study of the adsorption of oxygen molecules on individual semiconductiong single-walled carbon nanotubes at ambient conditions reveals that the adsorption is physisorption, the resistance without O2 increases by approximately two orders of magnitude as compared to that with O2, and the sensitive response is due to the pinning of the Fermi level near the top of the valence band of the tube, resulting from impurity states of O2 appearing above the valence band.

Thermoelectric power of graphene as surface charge doping indicator

We report on simultaneous thermoelectric power and four-probe resistance measurements of chemical vapor deposition grown graphene during a degas process, as well as in exposure to various gases. For all investigated samples, a dramatic change in thermoelectric power was observed and found to be sensitive to the gas molecule charge doping on the surface of graphene. The observed p-type behavior under ambient conditions supports an electrochemical charge transfer mechanism between the graphene and oxygen redox couple, while the n-type behavior under degassed conditions is ascribed to the electron doping caused by the surface states of the SiO2/Si substrate.

Electrostatic deposition of graphene in a gaseous environment: a deterministic route for synthesizing rolled graphenes?

The synthesis of single-wall carbon nanotubes of desired diameters and chiralities is critical to the design of nanoscale electronic devices with desired properties. The existing methods are based on self-assembly, therefore lacking control over the diameters and chiralities. The present work reports a direct route for rolling graphene. Specifically, we found that the electrostatic deposition of graphene yielded: (i) flat graphene layers under high vacuum (10(-7) Torr), (ii) completely scrolled graphene under hydrogen atmosphere, (iii) partially scrolled graphene under nitrogen atmosphere, and (iv) no scrolling for helium atmospheres. Our study shows that the application of the electrostatic field facilitates the rolling of graphene sheets exposed to appropriate gases and allows the rolling of any size of graphene. The technique proposed here, in conjunction with a technique that produces graphene nanoribbons of uniform widths, will have significant impact on the development of carbon nanotube based devices. Furthermore, the present technique may be applied to obtain tubes/scrolls of other layered materials.

Recent advances in synthesis, properties, and applications of phosphorene

Since its first fabrication by exfoliation in 2014, phosphorene has been the focus of rapidly expanding research activities. The number of phosphorene publications has been increasing at a rate exceeding that of other two-dimensional materials. This tremendous level of excitement arises from the unique properties of phosphorene, including its puckered layer structure. With its widely tunable band gap, strong in-plane anisotropy, and high carrier mobility, phosphorene is at the center of numerous fundamental studies and applications spanning from electronic, optoelectronic, and spintronic devices to sensors, actuators, and thermoelectrics to energy conversion, and storage devices. Here, we review the most significant recent studies in the field of phosphorene research and technology. Our focus is on the synthesis and layer number determination, anisotropic properties, tuning of the band gap and related properties, strain engineering, and applications in electronics, thermoelectrics, and energy storage. The current needs and likely future research directions for phosphorene are also discussed.