Diameter-controlled solid-phase seeding of germanium nanowires: structural characterization and electrical transport properties

Despite the huge progress recently made in understanding the phenomena of metal-promoted growth of one-dimensional (1D) semiconductors, the controlled formation of small diameter semiconductor nanowires is still challenging. Liquid growth promoters, such as the low melting Au/Ge eutectic, allow control of the aspect ratio, diameter, and structure of 1D crystals via external parameters, such as precursor feedstock, temperature, and operating pressure. However, the incorporation of metal atoms during the growth process, size variations of the nanowires due to agglomeration of the nucleating metal seeds, and surface diffusion of Au via the vapor–liquid–solid route have been reported. Here, we detail the influence of solid growth seeds, such as NiGe2 formed from Ni nanoparticles, on the lateral dimensions of Ge nanowires grown using a supercritical fluid growth process. Beneficial control over the mean nanowire diameter, in the sub-20 nm regime, with a predominantly ⟨110⟩ growth direction and low structural defect concentration was obtained using Ni seeds. In addition, the effect of prealloying of Ni–Fe films for the growth of Ge nanowires was investigated, which leads to a bimodal nanowire distribution. Electrical characterization performed on single nanowire devices showed p-type behavior for Ge nanowires grown from Ni and Ni/Fe seeds. Determination of resistivities, majority carrier concentrations, and mobilities suggest significant doping of the Ge nanowires by Ni when grown via a supercritical fluid–solid–solid (SFSS) mechanism

Contact resistivity and suppression of Fermi level pinning in side-contacted germanium nanowires

Electrical properties of contact-interfaces in germanium nanowire field effect transistor devices are studied. In contrast to planar bulk devices, it is shown that the active conduction channel and gate length extend between and underneath the contact electrodes. Furthermore, direct scaling of contact resistivity and Schottky barrier height with electrode metal function is observed. The associated pinning parameter was found to be γ=0.65 ± 0.03γ=0.65 ± 0.03, which demonstrates a significant suppression of Fermi level pinning in quasi-one-dimensional structures

In operandi observation of dynamic annealing: a case study of boron in germanium nanowire devices

We report on the implantation of boron in individual, electrically contacted germanium nanowires with varying diameter and present a technique that monitors the electrical properties of a single device during implantation of ions. This method gives improved access to study the dynamic annealing ability of the nanowire at room temperature promoted by its quasi-one-dimensional confinement. Based on electrical data, we find that the dopant activation efficiency is nontrivially diameter dependent. As the diameter decreases, a transition from a pronounced dynamic-annealing to a radiation-damage dominated regime is observed

Germanium nanowire synthesis from fluorothiolate-capped gold nanoparticles in supercritical carbon dioxide

Ge nanowires seeded from Au nanoparticles capped with fluorothiolate ligands were synthesized in supercritical carbon dioxide (sc-CO2) by the thermal decomposition of diphenylgermane (DPG) at a temperature of 380 °C and a pressure of 25.7 MPa. Both perfluorinated and semifluorinated capped Au nanoparticles acted as effective catalysts for growing Ge nanowires, with mean diameters of 11 nm (σ = 2.8) and 14 nm (σ = 3.5), respectively. The mean diameter of the Ge nanowires grown from the fluorous-capped Au nanoparticles were considerably smaller than those synthesized from dodecanethiol-capped nanoparticles in sc-toluene, under the same reaction conditions, i.e., 28 nm and σ = 10.3. Differences in the ligand conformations on the surface of the Au nanoparticles and phase separation of the fluorocarbon/CO2 and hydrocarbon/toluene systems gave rise to greater steric stabilization of the fluorous-capped Au nanoparticles in CO2, resulting in small diameter nanowires with a relatively narrow size distribution. Electrical analysis of the nanowires showed them to be p-type (hole) semiconductors

Germanium nanowire synthesis from fluorothiolate-capped gold nanoparticles in supercritical carbon dioxide

Ge nanowires seeded from Au nanoparticles capped with fluorothiolate ligands were synthesized in supercritical carbon dioxide (sc-CO2) by the thermal decomposition of diphenylgermane (DPG) at a temperature of 380 °C and a pressure of 25.7 MPa. Both perfluorinated and semifluorinated capped Au nanoparticles acted as effective catalysts for growing Ge nanowires, with mean diameters of 11 nm (σ = 2.8) and 14 nm (σ = 3.5), respectively. The mean diameter of the Ge nanowires grown from the fluorous-capped Au nanoparticles were considerably smaller than those synthesized from dodecanethiol-capped nanoparticles in sc-toluene, under the same reaction conditions, i.e., 28 nm and σ = 10.3. Differences in the ligand conformations on the surface of the Au nanoparticles and phase separation of the fluorocarbon/CO2 and hydrocarbon/toluene systems gave rise to greater steric stabilization of the fluorous-capped Au nanoparticles in CO2, resulting in small diameter nanowires with a relatively narrow size distribution. Electrical analysis of the nanowires showed them to be p-type (hole) semiconductors

Fingerprints of a size-dependent crossover in the dimensionality of electronic conduction in Au-seeded Ge nanowires

We studied the electrical transport properties of Au-seeded germanium nanowires with radii ranging from 11 to 80 nm at ambient conditions. We found a non-trivial dependence of the electrical conductivity, mobility and carrier density on the radius size. In particular, two regimes were identified for large (lightly doped) and small (stronger doped) nanowires in which the charge-carrier drift is dominated by electron-phonon and ionized-impurity scattering, respectively. This goes in hand with the finding that the electrostatic properties for radii below ca. 37 nm have quasi one-dimensional character as reflected by the extracted screening lengths

Diameter-controlled solid-phase seeding of germanium nanowires: structural characterization and electrical transport properties

Despite the huge progress recently made in understanding the phenomena of metal-promoted growth of one-dimensional (1D) semiconductors, the controlled formation of small diameter semiconductor nanowires is still challenging. Liquid growth promoters, such as the low melting Au/Ge eutectic, allow control of the aspect ratio, diameter, and structure of 1D crystals via external parameters, such as precursor feedstock, temperature, and operating pressure. However, the incorporation of metal atoms during the growth process, size variations of the nanowires due to agglomeration of the nucleating metal seeds, and surface diffusion of Au via the vapor–liquid–solid route have been reported. Here, we detail the influence of solid growth seeds, such as NiGe2 formed from Ni nanoparticles, on the lateral dimensions of Ge nanowires grown using a supercritical fluid growth process. Beneficial control over the mean nanowire diameter, in the sub-20 nm regime, with a predominantly ⟨110⟩ growth direction and low structural defect concentration was obtained using Ni seeds. In addition, the effect of prealloying of Ni–Fe films for the growth of Ge nanowires was investigated, which leads to a bimodal nanowire distribution. Electrical characterization performed on single nanowire devices showed p-type behavior for Ge nanowires grown from Ni and Ni/Fe seeds. Determination of resistivities, majority carrier concentrations, and mobilities suggest significant doping of the Ge nanowires by Ni when grown via a supercritical fluid–solid–solid (SFSS) mechanism

Diameter-driven crossover in resistive behaviour of heavily doped self-seeded germanium nanowires

The dependence of the resistivity with changing diameter of heavily-doped self-seeded germanium nanowires was studied for the diameter range 40 to 11 nm. The experimental data reveal an initial strong reduction of the resistivity with diameter decrease. At about 20 nm a region of slowly varying resistivity emerges with a peak feature around 14 nm. For diameters above 20 nm, nanowires were found to be describable by classical means. For smaller diameters a quantum-based approach was required where we employed the 1D Kubo–Greenwood framework and also revealed the dominant charge carriers to be heavy holes. For both regimes the theoretical results and experimental data agree qualitatively well assuming a spatial spreading of the free holes towards the nanowire centre upon diameter reduction

Degree of functionalisation dependence of individual Raman intensities in covalent graphene derivatives

Covalent functionalisation of graphene is a continuously progressing field of research. The optical properties of such derivatives attract particular attention. In virtually all optical responses, however, an enhancement in peak intensity with increase of sp3 carbon content, and a vanishing of the peak position shift in monolayer compared to few-layer systems, is observed. The understanding of these seemingly connected phenomena is lacking. Here we demonstrate, using Raman spectroscopy and in situ electrostatic doping techniques, that the intensity is directly modulated by an additional contribution from photoluminescent π-conjugated domains surrounded by sp3 carbon regions in graphene monolayers. The findings are further underpinned by a model which correlates the individual Raman mode intensities to the degree of functionalisation. We also show that the position shift in the spectra of solvent-based and powdered functionalised graphene derivatives originates predominantly from the presence of edge-to-edge and edge-to-basal plane interactions and is by large functionalisation independent