C. S. Guo

Tunable Electrical Properties of Silicon Nanowires via Surface-Ambient Chemistry

p-Type surface conductivity is a uniquely important property of hydrogen-terminated diamond surfaces. In this work, we report similar surface-dominated electrical properties in silicon nanowires (SiNWs). Significantly, we demonstrate tunable and reversible transition of p(+)-p-i-n-n(+) conductance in nominally intrinsic SiNWs via changing surface conditions, in sharp contrast to the only p-type conduction observed on diamond surfaces. On the basis of Si band energies and the electrochemical potentials of the ambient (pH value)-determined adsorbed aqueous layer, we propose an electron-transfer-dominated surface doping model, which can satisfactorily explain both diamond and silicon surface conductivity. The totality of our observations suggests that nanomaterials can be described as a core-shell structure due to their large surface-to-volume ratio. Consequently, controlling the surface or shell in the core-shell model represents a universal way to tune the properties of nanostructures, such as via surface-transfer doping, and is crucial for the development of nanostructure-based devices.

Surface Passivation and Transfer Doping of Silicon Nanowires

One-dimensional nanomaterials are expected to play a key role in future nanotechnology, in addition to providing model systems to demonstrate the unique characteristics of nanoscale effects. Silicon nanowires (SiNWs) in particular are potentially very attractive, given the central role of Si in the semiconductor industry, and are being extensively studied. A SiNW for use in nanodevices is composed of three sections: SiNW core, surface passivant, and adsorbates or interface compounds. A unique way to modulate the transport properties of SiNWs could depend on the individual sections. Volume doping is a conventional method to control conductivity. In volume doping, impurity atoms are introduced into the crystal lattice in the SiNW core by an in situ process during growth, 5] ion implantation, and related methods. However, volume doping for SiNWs has inherent disadvantages, such as poor controllability and destructive processing. Interestingly, the conductivity of amorphous Si films was found to be sensitive to adsorbates, which indicates the importance of the surface of low-dimensional systems in determining the electrical properties of materials. The large surface-to-volume ratio of SiNWs could potentially be important in influencing their transport properties. Its effect could be exploited through SiNW functionalization. Indeed, recent studies of SiNW-based chemical sensors 10] find strong conductivity responses of SiNWs to environmental conditions. Other relevant observations include conductivity modification by adsorbents in the hydrogen-terminated (H-terminated) surfaces of diamond crystals, conductivity determination by surface states in nanoscale thin silicon-on-insulator (SOI) systems, and conductivity enhancement of hydrogenated SiNWs in air and recovery through vacuum or gas purging. Thus, the possibility to modulate the conductivity of SiNWs using surface effects is promising. The ease of such an approach, economically and nondestructively, would offer a unique advantage for use of SiNWs in device fabrication. However, the success of this approach will depend on its controllability and repeatability, and most importantly on the understanding of the mechanisms of the surface effect on SiNWs. Herein, we present a new doping approach, namely surface passivation doping, built on the known surface transfer doping and based on extensive first-principles theoretical investigations and systematic experiments on the surface effects of SiNWs. We also elucidate the involved mechanism and provide better understanding to predetermine the electrical properties of nanomaterials. Surface hydrogen termination is a natural consequence of the hydrogen fluoride treatment of SiNWs. To reveal the role of hydrogen termination in conductivity, we first performed first-principles calculations based on density functional theory (DFT) with an efficient SIESTA code. 15] We adopted popularly used basis sets with double zeta and polarization functions and the Lee–Yang–Parr functional of generalized gradient approximation. We collected atomic charges from a Mulliken population analysis based on DFT calculation, which gave a reasonable charge distribution, as verified using a water molecule ( 0.46 j e j charges on the oxygen atom and 0.23 j e j charges on each hydrogen atom). Interestingly, we obtained extra charges of 0.06 j e j on average on each surface hydrogen atom of the H-terminated SiNWs (H-SiNWs). Clearly, the partial negative charge on the hydrogen atom is due to the higher electronegativity of the hydrogen atom compared to that of the silicon atom (2.2 vs. 1.9). This partial electron transfer from the silicon core to the surface hydrogen is negligible for bulk silicon but is significant for surface-dominated SiNWs whose carrier concentration could be considerably modified, as is estimated below. Assuming each surface silicon atom is terminated by two hydrogen atoms on average, we can calculate the total number of electrons trapped on the terminating hydrogen atoms using Equation (1):

Diameter-dependent spin polarization of injected carriers in carbon-doped zigzag boron nitride nanotubes

The diameter-dependent spin polarization of zigzag (n, 0) boron nitride nanotubes (5⩽n⩽10) with two carbon atoms substituting one boron atom and one nitrogen atom was investigated using first principles calculations. The spin polarization of the injected carriers is found in the tubes with larger diameters (n⩾7) and in a hexagonal boron nitride layer, but not in those with smaller diameters, attributable to the destruction of the π electronic structure rigidity.

An energetic stability predictor of hydrogen-terminated Si nanostructures

We present a linear relationship between the cohesive energies and the H/Si ratio for hydrogen-terminated Si semiconductor nanostructures based on our model analysis and first-principles calculations. The H/Si ratio is shown to be a universal predictor of the nanostructure’s energetic stability and allows easily searching of magic numbers in Si quantum dots. Our findings substantially improve the understanding of nanostructure stability and make practical the prediction of structural properties.

Prediction of surface passivation doping of silicon nanowires with phosphorus

We report a prediction of enhanced surface passivation doping effect in silicon nanowires (SiNWs) by phosphorus adsorption based on first-principles calculations. Recent theoretical and experimental studies all showed that hydrogen-passivated SiNWs present typical p-type characteristic due to charge transfer between the surface passivant and the SiNW core. Here, we show that a phosphorus-passivated SiNW with a moderate diameter facilitates improved hole generation in the core and efficient separation of electron and hole, which may provide a practical avenue for fabricating low cost solar cells with high efficiency.

Silicon nanowires for high-specificity and high-selectivity sensors under low-frequency scanning

The high specificity and selectivity of H–Si nanowire bundles, which are single crystalline and composed of pure Si without oxygen, for detecting water (peak at 12 Hz) and ethanol (peak at 70 Hz) in their mixture are measured by a frequency scanning test. The signal amplitude deduced between the work channel and the reference channel {[(VR-VS)/VR]×100%} is defined as the impedance recorded under different scanning frequencies.