Atomic nanofabrication using an ytterbium atomic beam

Abstract

Highly parallel and periodically narrow lines of ytterbium (Yb) atoms were successfully produced on a substrate using a near resonant laser light and direct-write atomic nanofabrication. Yb atoms are a promising material for nanofabrication using atom optics particularly due to their electric conductivity, the laser wavelength required for their manipulation, and the vapor pressure required for their fabrication. Collimated ¹⁷⁴Yb atoms were channeled into the nodes of an optical standing wave with dipole force and then deposited onto a substrate. We clearly observed a grating pattern of Yb atoms fabricated on a substrate with a line separation of approximately 200 nm after examining the surface of the substrate with an atomic force microscope. This is the first demonstration of nanofabrication using the atom-optical approach with Yb atoms.     

Analysis of femtosecond laser surface patterning on bulk single-crystalline diamond

Preliminary work is reported on 2-D and 3-D microstructures written directly with a Yb:YAG 1026?nm femtosecond (fs) laser on bulk chemical vapour deposition (CVD) single-crystalline diamond. Smooth graphitic lines and other structures were written on the surface of a CVD diamond sample with a thickness of 0.7?mm under low laser fluences. This capability opens up the opportunity for making electronic devices and micro-electromechanical structures on diamond substrates. The fabrication process was optimised through testing a range of laser energies at a 100?kHz repetition rate with sub-500?fs pulses. These graphitic lines and structures have been characterised using optical microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy. Using these analysis techniques, the formation of sp² and sp³ bonds is explored and the ratio between sp² and sp³ bonds after fs laser patterning is quantified. We present the early findings from this study and characterise the relationship between the graphitic line formation and the different fs laser exposure conditions.     

Characterization on pulsed laser deposited nanocrystalline ZnO thin films

Nanocrystalline zinc oxide thin films were deposited on glass and silicon substrates by using pulsed laser deposition at different laser energy densities (1.5, 2, and 3 J/cm²). The film thickness, surface roughness, composition, optical and structural properties of the deposited films were studied using an α-step surface profilometer, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), optical transmittance, and X-ray diffraction (XRD), respectively. The film thickness was calculated as 244 nm. AFM analysis shows that the root-mean-square roughness increases with increasing laser energy density. XPS analysis shows that the interaction of zinc with oxygen atoms is greatly increased at high laser energy density. In the optical transmittance spectra, a shift of the absorption edge towards higher wavelength region confirms that the optical band gap increases with an increase in laser energy density. The particle size of the deposited films was measured by XRD, it is found to be in the range from 7.87 to 11.81 nm. It reveals that the particle size increases with an increase in laser energy density.     

Selective Nanoscale Fabrication with Roughness Reduction of Polycrystalline Silicon by Physically Induced Fluorine Bonds Using Atomic Force Microscope

Polycrystalline silicon (poly-Si) is widely used as a gate layer in integrated circuits, transistors, and channels through nanofabrication. Nanoremoval and roughness control are required for nanomanufacturing of various electronic devices. Herein, a nanoscale removal method is developed to overcome the limitations of microcracks, complex procedures, and time-consuming conventional fabrication and lithography methods. The method is implemented with a mechanically induced poly-Si phase transition using atomic force microscope (AFM). Mechanical force induces the covalent bonds between silicon and fluorine atoms which cause the phase transition of poly-Si. Then, the bond structure of the Si molecules is weakened and selectively removed by nano-Newton-scale force using AFM. A selective nanoscale removal with roughness control is implemented in 0.5 mM TBAF solution after mechanical force (43.58–58.21 nN) is applied. By the magnitude of nano-Newton force, the removal depth of poly-Si is controlled from 2.66 to 21.52 nm. Finally, the nanoscale fabrication on poly-Si wafer is achieved. The proposed nanoremoval mechanism is a simple fabrication method that provides selective, nanoscale, and highly efficient removal with roughness control.     

Effect of oblique-angle deposition on early stage of Fe-Si growth

We have investigated the initial stage of Fe growth on an Si substrate during reactive oblique-angle deposition (ROAD) at 470 °C by means of atomic force microscopy, reflection high-energy electron diffraction, and high-resolution Rutherford backscattering spectroscopy. During deposition along the normal direction, many Si atoms are displaced from their lattice positions because of reactions with the deposited Fe. However, for ROAD, the number of displaced Si atoms decreases significantly along with a selective growth of nanoislands with diameters of a few 10 nm. Evidently, the local nucleation processes required for iron silicide formation are modified by the geometrical deposition conditions.     

Investigation of continuous-wave and Q-switched microchip laser characteristics of Yb:YAG ceramics and crystals

Continuous-wave and passively Q-switched microchip laser performance of Yb:YAG ceramics and single-crystals was investigated. Highly efficient continuous-wave Yb:YAG laser performance was observed at 1030 nm and 1049 nm for both Yb:YAG ceramics and crystals with different transmissions of output couplers. The laser performance of Yb:YAG ceramic is comparable to that of Yb:YAG single crystal. Meanwhile, the laser performance of laser-diode pumped Yb:YAG/Cr⁴⁺:YAG all-ceramics- and all-crystals-combination passively Q-switched microchip lasers were investigated. Sub-nanosecond laser pulses with peak power over 150 kW were obtained with different Yb:YAG/Cr⁴⁺:YAG combinations. Linearly polarized laser was observed in Yb:YAG/Cr⁴⁺:YAG all-crystals combination and circular polarized laser was obtained in Yb:YAG/Cr⁴⁺:YAG all-ceramics combination. The best laser performance was obtained with Yb:YAG/Cr⁴⁺:YAG all-crystals combination.     

Epitaxial growth of anatase TiO2 thin films on LaAlO3(100) prepared using pulsed laser deposition

Titanium dioxide thin films were grown on a lattice-matched LaAlO₃(100) surfaces using pulsed laser deposition (PLD) in oxygen atmosphere. The films were characterized using X-ray diffraction (XRD), reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM). The crystal structure of all the films was anatase. Preferred oriented films with a c-axis normal to the substrate surface were obtained. RHEED analysis also revealed that the films had the preferential in-plane orientation, demonstrating that anatase films were epitaxially grown on the substrate. The flatness of the films depended on their growth conditions and thickness.     

Static and Dynamic Properties of Multicomponent Bose–Einstein Condensates of Ytterbium Atoms

We consider theoretically static and dynamic properties of multicomponent Bose–Einstein condensates (BECs) composed of a mixture of ¹⁷⁴Yb and ¹⁷⁶Yb atoms. The condensate of ¹⁷⁶Yb atoms has an attractive interaction, collapsing above a certain critical particle number. This criterion is modified by the presence of another repulsive condensate of ¹⁷⁴Yb atoms. We discuss the stability condition of this condensate mixture and collapsing dynamics due to the instability.      

Quantum Degenerate Fermi Gases of Ytterbium Atoms

We performed evaporative cooling for dilute gases of ytterbium (Yb) isotopes in a crossed optical dipole trap and successfully cooled two fermionic and two bosonic species down to quantum degenerate regime, following the previous realization of Bose-Einstein condensation (BEC) in ¹⁷⁴Yb. The elastic collision rate of fermionic ¹⁷³Yb atoms with 6 spin components was found to be large enough to carry out efficient evaporation, which enables us to cool the atoms down to 0.6 T _F , where T _F is the Fermi temperature. In this regime, a plunge of evaporation efficiency was observed as an effect of the Fermi degeneracy. The other fermionic isotope ¹⁷¹Yb was cooled down to the temperature below T _F by sympathetic cooling with bosonic ¹⁷⁴Yb atoms. The sympathetic cooling technique has also been applied to ¹⁷⁴Yb-¹⁷⁶Yb Bose-Bose mixture. We have observed almost pure BEC of ¹⁷⁴Yb and the bimodal distribution of ¹⁷⁶Yb, showing the formation of BEC-BEC mixture. Moreover, we performed evaporative cooling of ¹⁷⁰Yb atoms and realized the BEC.     

AFM针尖耦合激光加工PMMA薄膜的加工重复性

利用飞秒激光与原子力显微镜的针尖部分耦合进行加工,是有望突破衍射极限,实现多隧道结加工的技术。本文在室温大气环境下,利用微焦级别脉宽为130fs、波长800nm的飞秒激光照射镀有金薄膜的原子力显微镜的探针针尖,使其耦合产生局域场加强效应在PMMA薄膜表面加工出光栅状纳米图形。通过最大残差法计算了条纹高度和宽度的重复性百分比,计算得到高度和宽度的最大残差值分别为6.3%和2.9%。结果表明在文中所提供的加工条件下,原子力显微镜针尖耦合激光是一种有潜力的激光加工方法。     

Design and fabrication of label-free biochip using a guided mode resonance filter with nano grating structures by injection molding process

This paper introduces technology to fabricate a guided mode resonance filter biochip using injection molding. Of the various nanofabrication processes that exist, injection molding is the most suitable for the mass production of polymer nanostructures. Fabrication of a nanograting pattern for guided mode resonance filters by injection molding requires a durable metal stamp, because of the high injection temperature and pressure. Careful consideration of the optimized process parameters is also required to achieve uniform sub-wavelength gratings with high fidelity. In this study, a metallic nanostructure pattern to be used as the stamp for the injection molding process was fabricated using electron beam lithography, a UV nanoimprinting process, and an electroforming process. A one-dimensional nanograting substrate was replicated by injection molding, during which the process parameters were controlled. To evaluate the geometric quality of the injection molded nanograting patterns, the surface profile of the fabricated nanograting for different processing conditions was analyzed using an atomic force microscope and a scanning electron microscope. Finally, to demonstrate the feasibility of the proposed process for fabricating guided mode resonance filter biochips, a high-refractive-index material was deposited on the polymer nanograting and its guided mode resonance characteristics were analyzed.     

Photoelectron spectra of Yb-Si(100) heterointerfaces formed at room temperature

The laws of formation of the heterointerfaces formed upon ytterbium (Yb) deposition at room temperature (T=300 K) onto a (2×1)-reconstructed Si(100) surface were studied by methods of photoelectron spectroscopy (PES) with excitation by synchrotron radiation and low-energy electron diffraction (LEED). In the range of submonolayer surface coverages θ, the adsorption of Yb atoms leads to destruction of the surface dimers. This is accompanied by the formation of an amorphous film consisting of mixed Yb and Si atoms. For θ>1.5, a partly ordered metallic ytterbium film containing dissolved Si atoms is formed on the substrate surface. In all stages of deposition, there is charge transfer from Yb to Si atoms, which is manifested by a shift of the Si 2p core level. This shift is especially pronounced (reaching 1.5 eV) for silicon atoms dissolved in the metallic film.     

Fabrication and characterization of epitaxial Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>TiO<Subscript>3</Subscript>/LaNiO<Subscript>3 </Subscript>heterostructures

Epitaxial Ba0.6Sr0.4TiO3 (BST)/LaNiO3 (LNO) heterostructures were fabricated on LAO (100) substrates using pulsed laser deposition (PLD). Their structural properties were investigated by X-ray diffraction (XRD). The θ–2θ scans showed single crystalline BST and LNO layers with a (100) orientations perpendicular to the substrate plane. Phi scans (ϕ) on the (220) plane of BST layer indicated that the films have two in-plane orientations with respect to the substrate. The atomic force microscope (AFM) surface morphologies showed a smooth and crack-free surface with the average grain size of 55 nm and the root-mean-square (RMS) of 4.53 nm for BST films. Capacitance–voltage curves are measured. From the capacitance, a dielectric constant of 762, tunabilty of 82.81% and loss tangent of 0.032 are obtained. The current–voltage curve shows that the leakage current is 2.41 × 10⁻⁷ A/cm² under an applied voltage of 2 V.     

Nanoporous cluster-assembled WOx films prepared by radio-frequency assisted laser ablation

The influence of substrate temperature and ambient gas pressure-composition on the characteristics of WO_x films synthesized by radio-frequency assisted pulsed laser deposition (RF-PLD) are studied with the aim to obtain nanostructured films with large surface area that appear promising for gas sensing applications. A tungsten target was ablated both in chemically reactive molecular oxygen at 5 Pa and in a mixed oxygen-helium atmosphere at 700 Pa. Corning glass was used as the substrate, at 473, 673 and 873 K. Other deposition parameters such as laser fluence (4.5 J/cm²), laser wavelength (355 nm), radio-frequency power (150 W), and target to substrate distance (4 cm) were kept fixed. The sensitivity on the deposition parameters of roughness, morphology, nanostructure and bond coordination of the deposited films were analysed by atomic force microscopy, scanning electron microscopy, transmission electron microscopy and micro-Raman spectroscopy. The role of the investigated process parameters to nanoparticle formation and to the development of an extended nanostructure is discussed.     

Effect of the ion beam current density on the formation of implanted metal nanoparticles in a dielectric matrix

The effect of the ion beam current density, varied within 4–15 μA/cm², on the formation of metal nanoparticles in a subsurface layer of SiO² substrates implanted with 30-keV Ag⁺ ions to a dose of 5×10 16 cm⁻² was studied by optical spectroscopy and atomic force microscopy techniques. An increase in the ion beam current density leads to the formation of nanoparticles of a greater size as a result of the glass substrate heating and due to an increase in the diffusion mobility of implanted silver atoms. These results suggest the possibility of controlling the dimensions of implanted nanoparticles in dielectrics by means of variation of the ion beam current density during the process.     

An Atomistic Tomographic Study of Oxygen and Hydrogen Atoms and their Molecules in CVD Grown Graphene

The properties and growth processes of graphene are greatly influenced by the elemental distributions of impurity atoms and their functional groups within or on the hexagonal carbon lattice. Oxygen and hydrogen atoms and their functional molecules (OH, CO, and CO₂) positions' and chemical identities are tomographically mapped in three dimensions in a graphene monolayer film grown on a copper substrate, at the atomic part‐per‐million (atomic ppm) detection level, employing laser assisted atom‐probe tomography. The atomistic plan and cross‐sectional views of graphene indicate that oxygen, hydrogen, and their co‐functionalities, OH, CO, and CO₂, which are locally clustered under or within the graphene lattice. The experimental 3D atomistic portrait of the chemistry is combined with computational density‐functional theory (DFT) calculations to enhance the understanding of the surface state of graphene, the positions of the chemical functional groups, their interactions with the underlying Cu substrate, and their influences on the growth of graphene.     

Capabilities of femtosecond laser ablation inductively coupled plasma mass spectrometry for depth profiling of thin metal coatings

The capabilities of ultraviolet femtosecond laser ablation inductively coupled plasma mass spectrometry (UV-fs-LA-ICPMS) for depth profile analysis of thin metal coatings were evaluated. A standard sample consisting of a single Cr thin layer of 500 nm +/- 5% on a Ni substrate was used. A fast washout was obtained by a high-efficiency aerosol dispersion ablation cell (V approximately 1 cm3), which allowed single-shot analysis with increased depth resolution. Laser ablation was performed in helium at atmospheric pressure conditions. A laser repetition rate of 1 Hz and low laser fluence (<0.5 J/cm2) were used. Very low ablation rates (<10 nm/pulse) were determined by atomic force microscopy (AFM). Information about the crater geometry and morphology was investigated using scanning electron microscopy and AFM. The depth resolution, calculated via the maximum slope of the tangent in the layer interface region, was smaller than 300 nm. Our data indicate that UV-fs-LA-ICPMS represents a powerful combination of high lateral and depth resolution for the analysis of thin metal coatings. Moreover, an overall ion yield, defined as the ratio of detected ions and ablated atoms, of approximately 5 x 10-5 was estimated for the chromium layer under the operating conditions chosen. The absolute amount of ablated material per laser pulse was approximately 1 pg, which corresponds to a detection limit of 180 microg/g.     

Realization of a mirror magneto-optical trap

Abstract

We present details for construction and the operation of a mirror magneto-optical trap for cooling and trapping rubidium atoms. For trap operation, only four input laser beams are needed in contrast to the normal six for a standard trap. In excess of 10⁸ atoms have been trapped with this arrangement, with the atomic ensemble only ～1mm from the surface of a reflective mirror. This trap is highly suited to studies of magnetic guiding and magnetic manipulation of cold atomic ensembles.     

Fabrication of LSS bottom electrode by PLD

Polycrystalline LaNiO₃/SrTiO₃/Si(100) (LSS) conducting substrates were fabricated by pulsed laser deposition (PLD) technique. LSS substrate is a potential candidate for the multiferroic materials for use as bottom electrode. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) equipped with EDX system, atomic force microscopy (AFM) and electrical resistivity were employed to characterize the films. Buffer layer SrTiO₃ (STO) deposited at 700 °C resulted in dense, smooth and with crack free features. XRD studies confirmed bi-crystalline [(100), (110)] growth of STO on Si(100) substrate. Deposition of bottom electrode LaNiO₃ (LNO) epitaxially followed the buffer layer. EDX analyses determined the chemical composition of the films. The role of oxygen partial pressure during deposition affecting the crystallinity and resistivity of the films was explored in detail. Atomic force microscopy revealed the atomic scale features of the films desirable for functional devices. Resistivity of the conducting film (LNO) was ∼10⁻⁴ Ω cm at room temperature. Thus it is demonstrated that LNO/STO/Si(100) is a suitable conducting substrate for growth of the multiferroic functional materials.     

Heteroepitaxy of zinc-blende SiC nano-dots on Si substrate by organometallic ion beam

The self-assembled SiC nano-dots were fabricated on Si(111) substrate at low-temperatures using the organometallic ion beam deposition technique. The single precursor of methylsilicenium ions (SiCH₃⁺) with the energy of 100 eV was deposited on Si(111) substrate at 500, 550 and 600 °C. The characteristics of the self-assembled SiC nano-dots were analyzed by reflection high-energy electron diffraction (RHEED), Raman spectroscopy and atomic force microscope (AFM). The RHEED patterns showed that the crystal structure of the SiC nano-dots formed on Si(111) substrate was zinc-blende SiC (3C-SiC) and it was heteroepitaxy. The self-assembled SiC nano-dots were like a dome in shape, and their sizes were the length of 200-300 nm and the height of 10-15 nm. Despite the low-temperature of 500 °C as SiC crystallization the heteroepitaxial SiC nano-dots were fabricated on Si(111) substrate using the organometallic ion beam.