Session L17: Structural and Optical Properties of Nanostructures

X-ray Scattering Studies of Monolayer Assembly of Zeolite Crystals

We characterized monolayer assemblies of zeolite crystals using x-ray reflectivity and diffuse scattering. They were prepared on Si wafers through two different types of molecular linkages, namely, through the direct linkage between the Si-tethered 3- chloropropyl (CP) groups and the surface hydroxyl groups of zeolites and through the linkage between zeolite- and Si- tethered CP groups via polyethylene imine (PEI) as the intermediate linker. X-ray reflectivity results clearly differentiated the two types of linkages and furthermore showed the thickness and the density of each component layer before and after the assembly of zeolite monolayers on substrates, demonstrating that this analytical technique can serve as a powerful tool to collect important information about the molecular linkages between the molecularly tethered microcrystals and substrates.   

Absorption and Photoluminescence in very small diameter Si Nanowires

Optical absorption and photoluminescence spectra on 3 sets of crystalline Si nanowires with most probable diameter 3.5 nm, 5.5 nm and 9 nm are presented. In the optical absorption spectra, apart from the direct gap absorption at Er1 $\sim $ 3.4 eV and Er2 $\sim $ 4.2 eV, we observed two additional strong peaks near 1.5 eV and $\sim $2.5 eV. The 3.4 and 4.2 eV peaks exhibit a weak but clear blue shift with decreasing wire diameter. Interestingly, the anomalous 1.5 and 2.5 eV peaks increase in intensity with decreasing nanowire diameter. It is also interesting that the 1.5 eV peak does not shift with decreasing wire diameter. This behavior leads us to tentatively assign this structure to Si-SiO2 interface states. On the other hand, the $\sim $2.5, 3.4 and 4.2 eV absorption bands exhibit a systematic blue shift with decreasing diameter. This behavior is consistent with a quantum confinement phenomenon. Structure in the photoluminescence is also observed at $\sim $1.5 and $\sim $2.4 eV. The origin of these low energy peaks will be described in terms of the band structure of Si nanowires.   

Interplay of Phonon Confinement and Thermal Phenomena on the 520 cm-1 Raman band in Very Small Diameter Silicon Nanowires

Results of Raman experiments that investigate the influence of laser flux and thermal anchoring on the asymmetric line profile $\sim $520 cm$^{-1}$ optical phonon scattering from small diameter Si nanowires are presented. At low laser flux $\Phi \quad \le $ 20$\mu $W/$\mu $m$^{2}$, the lineshape seems well described by a phenomenological lineshape function associated with phonon confinement due to Richter et al$^{1}$. However, at high laser flux $\ge $ 100 $\mu $W/$\mu $m$^{2}$, the Raman band takes on even higher asymmetry that is likely due to inhomogenous heating. The data at low and high laser flux can be explained quantitatively on the basis of fundamental Raman scattering theory. $^{1}$ H.$^{ }$ Richter, Z. P. Wang, and L. Ley, Solid State Communcations, \textbf{39}, 625 (1981) $^{\dag }$Work supported by the NSF NIRT program (DMR- 0304178).   

Thin film assemblies of silicon nanoparticles roll up into flexible nanotubes

We report on synthesis of flexible nanotubes made of a self-assembly of fluorescent silicon nanoparticles. When a colloidal dispersion of the Si nanoparticles in alcohol is submitted to an electric field, the particles are driven to one of the electrodes via eletrophoresis. We coat various surfaces with thin films of silicon particles. Upon drying, the film rolls up into uniform tubes. We used Atomic Force Microscopy (AFM) and a linear elasticity model to measure the young modulus of this film. It was found to be as flexible as rubber. These structures have potential applications for future enhanced biological recognition and sensing of toxins. Moreover, they are useful as catalysts, and in nano robotic applications.   

Sensitivity of dangling bonds to the presence of dopant atoms in hydrogen terminated silicon nanoclusters

Dangling bonds on the surfaces of semiconductor nanoparticles are expected to play an important role in the self-assembly of hybrid molecule-semiconductor nanoelectronic devices[G.P. Lopinski, D. D. M. Wayner, and R. A. Wolkow, Nature 406, 48 (2000)] and should also directly impact the electronic and transport properties of such nanostructures. We have studied the dangling bond on hydrogen terminated silicon nanoparticles, both analytically in the effective mass approximation and using a self-consistent Poisson-Schr\"odinger model that we previously developed.[Phys. Rev. B, to be published] This model allows us to make calculations on silicon structures containing many hundreds of silicon atoms, enabling us to explore (doped) silicon nanoparticles with dangling bonds. A dangling bond on a hydrogen terminated silicon surface is shown to behave qualitatively as an electronic acceptor, its energy level however depends on the occupation of the dangling bond state which in turn depends on the the temperature and strongly on doping of the silicon. We will present energies and wave functions for dangling bond states on different (doped) hydrogen terminated silicon surfaces.   

Formation of Micro Tubes from Strained SiGe/Si Heterostructures

We report the formation of arrays of micrometer-sized SiGe/Si tubes by releasing strained SiGe/Si heterostructures from substrates. The silicon oxide sacrificial layer is etched by hydrofluoric acid buffered with ammonia fluoride. Because of the dynamic curvature change of the bilayer, the etching process deviates from the conventional ransport-controlled regime to the kinetic-controlled regime. A slow symmetric and a fast asymmetric etching mode are identified. The fast mode is associated with asymmetric surface deformation. Large etch channels are induced and etching becomes reaction controlled. In the slow etching process, bilayers are symmetrically deformed and retain mostly the initial surface pattern. A crossover from the transport-controlled (symmetric) etching to kinetic-controlled (asymmetric) etching is observed when the size of the bilayers becomes much larger than the curvature radius. Dispersion in etch rate is directly related to the degree of asymmetry in surface deformation. Using a micro manipulator, SiGe/Si tubes are assembled onto a micro-strip line for radio-frequency characterizations.   

Optical and Electronic Characteristics of Germanium Quantum Dots Formed by Selective Oxidation of SiGe/Si-on-Insulator

A complementary metal-oxide-semiconductor (CMOS)-compatible method is proposed to form atomic-scale germanium (Ge) quantum dots ($<$10 nm) for application in single-electron (SE) devices or optical devices. The formation of Ge quantum dots is realized by the Ge atoms' segregation and agglomeration during thermal oxidation of Si$_{1-x}$Ge$_{x}$ alloys. The size and distribution of the Ge dots are determined by conditions of thermal oxidation process and Ge content in the alloys. The optical and electronic characteristics of Ge quantum dots were examined by $x$-ray diffraction, high-resolution transmission electron microscopy, cathodoluminescence spectroscopy, and spectroscopic ellipsometry. The dot size and crystallite morphology were strongly dependent on thermal oxidation conditions. Visible photoemissions from Ge dots were observed at room temperature and they exhibited pronounced blueshifts of peak energies with increasing oxidation time, which can be correlated to the change in dot size, shape, or crystalline structure transition. Compared to bulk Ge, the reduced refractive index and relevant blueshifts of band-structure critical points of Ge quantum dots, derived from spectroscopic ellipsometry, are also correlated to the nanocrystal size effects.   

Raman spectroscopy of free-standing Ge nanocrystals

Ge nanocrystals, with an average diameter of 5 nm, are grown in a silica matrix. Free-standing nanocrystals are obtained by selectively etching the oxide. Embedded nanocrystals experience considerable compressive stress relative to the bulk. The Raman line position of free-standing nanocrystals is redshifted by $\sim $6 cm$^{-1}$ relative to that of embedded nanocrystals, indicating relief of the compressive stress. Mixed surface/bulk vibrational modes between 125 and 250 cm$^{-1}$ and surface modes below 125 cm$^{-1}$ are observed by Raman spectroscopy on free-standing nanocrystals. These modes are not observed for the case of embedded nanocrystals. Exposed nanocrystals are stable under ambient atmospheric conditions after the formation of a thin, self-limiting native oxide layer. The effect of oxide layer thickness on the vibrational spectra of free-standing nanocrystals will be discussed. This work is supported in part by U.S. NSF Grant Nos. DMR-0109844 {\&} EEC-0085569 and in part by U.S. DOE under Contract No. DE-AC03-76F00098.   

Ge nanocrystals embedded in sapphire

Ge nanocrystals are formed in a sapphire matrix by ion implantation followed by thermal annealing. Transmission electron microscopy (TEM) of as-grown samples reveals that the nanocrystals are faceted. Notably, the matrix remains crystalline despite the large implantation dose and corresponding damage. Embedded nanocrystals experience up to 4 GPa of compressive stress relative to bulk, as measured by Raman spectroscopy of the zone center optical phonon. In contrast, nanocrystals embedded in silica are observed to be spherical and experience considerably lower stresses. Also, \textit{in situ} TEM reveals that nanocrystals embedded in sapphire melt very close to the bulk melting point (T$_{m}^{ }$= 936 $^{\circ}$C) whereas those embedded in silica exhibit a significant melting point hysteresis around T$_{m}$. This work is supported in part by U.S. NSF Grant Nos. DMR-0109844 {\&} EEC-0085569 and in part by U.S. DOE under Contract No. DE-AC03-76F00098.   

Structural and Optical Properties of Sn$_x$Ge$_{1-x}$ thin films and Quantum Dots

Sn$_{x}$Ge$_{1-x}$ layers and quantum dots (QDs) are of great interest as materials that could provide tunable direct band gaps, allowing completely group IV-based optoelectronic devices. These materials could be used in a wide range of applications such as emitters, infrared detectors, and thermophotovoltaics. However, substantial challenges remain in the growth and processing of these materials. We have grown Sn$_{x}$Ge$_{1-x}$ films by Molecular Beam Epitaxy (MBE), using low growth temperatures ($<$200$^{\circ}$C) in order to grow fully strained layers. X-ray diffraction, transmission electron microscopy, and Rutherford backscattering spectroscopy data indicate high-quality epitaxial films. Post-growth annealing was used to form QDs. Either QDs or quantum wires may be formed depending on annealing parameters. The effects of varying substrate temperature between 400C (wires) and 750C (QDs) on size and distribution of quantum structures were explored and will be discussed. Sn concentration (0-10{\%}) and film thickness (40nm - 200nm) were also varied. Optical properties probed by Fourier transform infrared spectroscopy (FTIR) will be presented. FTIR spectra clearly show the decrease in band gap of Sn$_{x}$Ge$_{1-x}$ layers with increasing Sn fraction up to 10{\%}. Photomodulated reflectance (PR) is another sensitive method for probing critical points in Sn$_{x}$Ge$_{1-x}$ band structure, and can detect both direct and indirect transitions. PR results for Sn$_{x}$Ge$_{1-x}$ layers will also be discussed.   

Raman Scattering from Surface Optic Phonons in Cylindrical and Rectangular Cross-sectional Semiconducting Nanowires†

Raman scattering from surface optic (SO) phonons has been observed and identified in cylindrical GaP and rectangular cross-section ZnS nanowires. We propose that the symmetry breaking mechanism which activates the SO phonon is a periodic modulation of the cross-sectional area along the nanowires. In the case of cylindrical GaP nanowires, Raman scattering from SO phonons in air at room temperature is observed at 394 cm-1, in between the first order longitudinal optic (LO) (401 cm-1) and transverse optic (TO) (367 cm-1), and downshift to 392 cm-1 in dichloromethane (?m=2.0) and 390 cm-1 in aniline (?m=2.56). Raman scattering from the ZnS nanowires in air at room temperature reveals a strong first-order LO mode at 346 cm-1 and two TO modes at 269 and 282 cm-1. The SO Raman band in ZnS is observed at 335 cm-1 in air, and downshifts to 328 cm-1 in dichloromethane and to 326 cm-1 in aniline. The position of the SO band in GaP and ZnS nanowires is consistent with a dielectric continuum model. Theoretical SO phonon dispersion for both cylindrical and rectangular cross-section nanowires is presented and compared to experiment. {\dag}This work was supported by the NSF NIRT program (DMR- 0304178).   

Fabrication of Magnetically Half-Coated Nanoparticles Using Molecular Beam Deposition

The top-down approach of building nanodevices can be combined with the bottom-up to create half-coated nanoparticles with well controlled magnetic, optical, electronic, and chemical properties. A type of half-coated nanoparticle particle that utilizes both optical and magnetic control is Magnetically Modulated Optical Nanoprobes (MagMOONs) (JN Anker, and R Kopelman, Appl. Phys. Lett., 82, 1102-1104 (2003).). MagMOONs emit modulated fluorescence or reflection intensities when externally manipulated by a magnetic field. We have fabricated functional MagMOONs by coating a thin layer of polycrystalline cobalt onto micro and nanospheres using molecular beam deposition (MBD). Additionally, vectorial magneto-optical Kerr effect was used to study the magnetization reversal of the coated spheres supported by a substrate. Hysteresis loop variations associated with the sphere size changes are observed and compared with the case of planar deposited films.   

Highly-Ordered Monolayers in a Hurry: Non-Equilibrium Pathways to High-Quality Superlattices

The formation of 2D superlattices has long been thought to occur at the end of evaporation when particle-particle, -solvent and -substrate interactions can induce spinodal phase separations. Surprisingly, recent experiments have shown that it is indeed possible to create highly-ordered nanocrystal monolayers by non-equilibrium processes such as drop drying. In these cases the monolayer self-assembly occurs on the liquid-air interface. We unravel details of the self-assembly mechanism by tracking the monolayer formation through its entire evolution using a combination of direct optical microscopy and transmission electron microscopy. Drop drying itself is a highly non-equilibrium phenomenon that is affected by solvent evaporation, mass transport inside the drop, drop geometry, environmental conditions, and other factors. Despite this complexity, we observed that highly-ordered monolayers form according to three different robust growth laws: linear, quadratic, and exponential in time. We delineate a phase diagram based on a simple geometric model that can be used to better understand the self-assembly of nanocrystal thin films from solution.   

Mechanical Properties of CdSe Tetrapods

CdSe tetrapods are novel nanoscale crystals with unique electric/optical properties, which makes them promising candidates for making nanocrystal based photovoltaic solar cells. However, their mechanical properties are still not well understood. We used atomic force microscopy to study their mechanical and electrical properties and try to discover the correlation between them. First of all, the AC mode images revealed that each arm of the tetrapods is about 150 nm long except the one that is along the surface normal (the vertical arm). They also showed that the tetrapods deposited on Si surfaces have already been pushed down by the capillary force caused by surface water layer. Additionally, the mechanical properties of CdSe Tetrapods were studied using force-volume technique. We were able to put the AFM tip right on the top of the vertical arm using this technique. We discovered that the tetrapods would undergo elastic deformation if the applied force was less than 52 nN. After we applied force more than 91 nN, the tetrapods would undergo plastic deformation and we started to observe the bending of the vertical arm. Applying a force more than 130 nN on top of the vertical arm would then completely destroy the tetrapods. Current Force Microscopy will be used to study the mechanical and electrical properties of tetrapods at the same time, which will be expected to give us more insight on the relationship between the electrical properties of CdSe tetrapods and their ``molecular'' structures.   

Library of Templates of Virtuall Small Clusters of Ga and In with As, P and V Atoms

Electronic energy level computations for small atomic clusters are feasible and provide a foundation for realization of a virtual (i.e., fundamental theory- based, computational) approach to synthesis of sub-nanoscale materials with pre-designed electronic and magnetic properties. In this study the Hartee-Fock (HF) and CI/CAS/MCSCF methods have been used to develop about 20 stable pre-designed and vacuum virtual clusters of Ga and In with As, P and V atoms that provide templates for experimental synthesis of the corresponding clusters in confinement and vacuum. The electronic energy level spectra (ELSs) and spin density distributions (SDDs) of the template clusters reflect influence of clusters' growth conditions, form and composition on formation and structure of their valence and conduction bands, the values of their optic transition energies, and collectivization of their spin density distributions. The ELSs and SDDs of the templates are compared and classified according to their possible use in development of sub-nanostructured materials for electronics, magneto-optics and spintronics.