Jaideep S. Kulkarni

Conductive films of ordered nanowire arrays

High-density, ordered arrays of germanium nanowires have been synthesised within the pores of mesoporous thin films (MTFs) and anodized aluminium oxide (AAO) matrices using a supercritical fluid solution-phase inclusion technique. Conductive atomic force microscopy (C-AFM) was utilised to study the electrical properties of the nanowires within these arrays. Nearly all of the semiconductor nanowires contained within the AAO substrates were found to be conducting. Additionally, each individual nanowire within the substrate possessed similar electrical properties demonstrating that the nanowires are continuous and reproducible within each pore. C-AFM was also able to probe the conductance of individual nanowires, 3–4 nm in diameter, within the MTF templates. The ability to synthesise ordered arrays of semiconducting nanowires is a key step in future ‘bottom-up’ fabrication of multi-layered device architectures for potential nanoelectronic and optoelectronic devices.

Size dependent thermal properties of embedded crystalline germanium nanowires

Here we report the size-dependent melting points of crystalline germanium nanowires confined within the pores of hexagonal mesoporous silica templates. A supercritical fluid deposition technique was used to form the nanowire–template composite materials and differential thermal analysis, coupled to thermal gravimetric analysis, was used to determine the melting points of the embedded Ge nanowires with mean diameters ranging from 22 to 85 A. The melting points of the Ge nanowires within the templates were found to be higher than the melting point of bulk germanium (937 °C), typically by 60 °C, and with a broad melting range (∼80 °C). Extended X-ray absorption fine structure (EXAFS) analysis of the Ge K-edge from the nanocomposite materials revealed a linear increase in the Ge–Ge nearest neighbour distance with decreasing nanowire diameter over the size range investigated. In all cases the Ge–Ge first shell distance in the nanowires was greater than in the bulk. This observed lattice expansion can be attributed to crystallographic deformation resulting from the strain imposed on the nanowires by the template. EXAFS studies also revealed an increase in the average number of oxygen atoms at the Ge/silica interface with decreasing diameter due to the increasing surface area. Interfacial effects and interactions at the nanowires/matrix interface are believed to delay the melting point of these systems.

Engineering the magnetic properties of Ge1−xMnx nanowires

Possible origins of room-temperature ferromagnetism in GeMn nanowires (NWs) are investigated. Arrays of Ge1−xMnx NWs and Ge∕Ge1−xMnx nanocables (NCs) (x=1%–5%) have been synthesized within the pores of anodized alumina oxide (AAO) membranes. The influence of annealing on the magnetic properties of Ge1−xMnx NWs is studied. The room-temperature ferromagnetism is preserved after the postfabrication annealing in inert atmosphere (Tann=750°C) demonstrating overall compatibility of Ge1−xMnx NWs with conventional complementary metal-oxide semiconductor technology. The role of oxygen in high-TC ferromagnetic ordering is investigated in double-phased NCs with a Ge sheath. Despite a barrier to oxygen migration from the AAO membrane, samples still display room-temperature ferromagnetism, hence, ruling out any significant role of oxygen in the explanation of the high TC in the system. The magnetic properties of the one-dimensional Ge1−xMnx nanostructures can be understood by considering interface related phenomena.

Spin dynamics in oriented ferromagnetic nanowires Ge0.99Co0.01

The magnetic properties of one-dimensional oriented nanowires Ge0.99Co0.01 grown in pores of anodized aluminum oxide membranes are investigate using ferromagnetic resonance spectroscopy. The electron spin resonance signals of the magnetically ordered cobalt subsystem and the charge-carrier subsystem are identified. It is revealed that the anisotropy field at 4 K is equal to 400 Oe and aligned parallel to the nanowire axis. The transverse relaxation time of spin waves at 4 K is estimated to be ∼10−10 s. It is shown that the magnetic properties of nanowires are predominantly determined by the ferromagnetism of Co and GeCo alloy clusters.

Structural investigation of germanium–cobalt core shell nanocable arrays

The compositional structure of high density arrays of coaxial nanocables, consisting of germanium nanowires surrounded by cobalt nanotube sheaths, within anodic aluminium oxide membranes, were investigated by various X-ray analysis techniques. The nanocable arrays were prepared using a supercritical fluid inclusion process, whereby the cobalt nanotube precursor was first deposited on the pore walls of the nanoporous membranes and subsequently filled with germanium to form coaxial nanocables. The composition and structure of both the as-synthesised (450 °C) and annealed (850 °C) nanostructures were investigated by X-ray diffraction at high angles (XRD), X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge spectroscopy (XANES).

Magnetic resonance in Ge0.99Mn0.01 nanowires

The contributions of several different subsystems to the magnetic properties of Ge0.99Mn0.01 nanowires are distinguished. The ferromagnetic resonance spectrum is found to have four components, two of which have the same temperature dependence and a Lorentzian shape. Presumably, these components correspond to the excitation of spin waves in the Mn3+ ion subsystem under the simultaneous influence of exchange and dipole-dipole interactions. There is also another Lorentzian-shaped component corresponding to resonance in the subsystem of localized Mn2+ centers. The fourth spectrum component has an asymmetric Dyson shape and is related to the resonance of mobile paramagnetic centers. A correlation is found between the temperature dependences of the spectral parameters of the magnetic resonances of the localized centers (Mn3+ and Mn2+ ions) and the charge carrier subsystem. This correlation indicates that the ferromagnetic exchange between the localized centers is due to carrier spin transport.