AVS2018 Session NS-ThP: Nanometer-scale Science and Technology Division Poster Session

Electrons with high kinetic energy (1-3 eV) can be generated in metals during surface reaction processes. These energetic electrons are called “hot electrons”. A way to detect these hot electrons is by using metal-semiconductor Schottky diode. It was proposed that enhanced light absorption with localized surface plasmon resonance results in amplified hot electron generation by utilizing Au/TiO₂ Schottky diodes. In this scheme, the surface morphology of the metal thin film was modified to a connected gold island structure that exhibits surface plasmons.[1,2]

To probe the enhanced hot electron flows by surface plasmon, we fabricated patterned Au islands on TiO₂ diodes using e-beam evaporator, [3] and measured the local photocurrent with the conductive probe atomic force microscopy under back illumination of the light. The gold pattern has triangle shape with the length of the hypotenuse of 150 nm and the thickness of 20 nm. We found that the photocurrent depends on the wavelength of laser, and the bias between Au and TiO₂. The photocurrent measured at the edge of the Au islands was higher than that on the flat area of Au islands. The result indicates the localized surface plasmon resonance leads to enhancement of hot electron flux.

Reference

1 Y . K. Lee, C. H. Jung, J. Park, H. Seo, G. A. Somorjai , and J. Y . Park , Nano Lett. 11, 4251 (2011).

2 H. Lee, Y. K. Lee, E. Hwang, and J. Y . Park, J. Phys. Chem. C. 118, 5650-5656 (2014).

3 H. Lee, Y. K. Lee, T. N. Van, and J. Y. Park, Appl. Phys. Lett. 103, 173103 (2013).

¹ Koike, K. et. al. Proc. SPIE 10586, Advances in Patterning Materials and Processes XXXV, 105861F (13 March 2018);

² Bunday, B. et. al. Proc. SPIE 10585, Metrology, Inspection, and Process Control for Microlithography XXXII, 105850I (22 March 2018);

³ Rosa, A. M. et al. IEEE 30th Symposium on Microelectronics Technology and Devices (SBMicro) (2015).

We report the deposition and alignment of semiconducting carbon nanotubes with the alternating electric field-directed dielectrophoresis (DEP) method and the fabrication of carbon nanotube-based electronic devices with the DEP-aligned semiconducting carbon nanotubes (CNTs). Semiconducting carbon nanotubes, which were dispersed ultrasonically in solutions, were deposited and aligned onto a pair of gold electrodes using the dielectrophoresis method. The DEP-aligned tubes were further fabricated into carbon nanotube field-transistors (CNTFETs) and CNTFET-based electronic devices such as CNT-based inverters and ring oscillators using the microfabrication techniques. The aligned carbon nanotubes and fabricated devices were imaged using the scanning electron microscope (SEM), and the electrical properties were measured from the fabricated devices using the semiconductor analyzer. The semiconducting CNTs achieved higher yield in the device fabrication, and the fabricated devices demonstrated excellent electrical properties.

Bacteria produce rotary filamentous appendages, known as flagella, for propulsion through their environment in response to various chemical signals. The flagella, of nanoscale width and of microscale length, can be easily isolated from the microorganisms at low cost and in large quantities. Once isolated, these nanofilaments of uniform size distribution can be deposited onto desired surfaces in controlled quantities and can act as novel templates for nanostructures. Flagella placed on a surface ahead of ionic, size-selected metallic cluster deposition could act as scaffolds in the construction of nanocluster networks. These organized nanocluster networks could then be used to investigate the various unique quantum and nanoscale properties exhibited by finite-size systems. These include enhanced surface plasmon resonance, catalytic applications, charge tunneling junctions, and Josephson current in potential superconducting arrays.

Direct laser writing has been established as a prototyping tool for the rapid fabrication of optical materials with nanometer-sized features. So far, however, highly reflective photonic crystals have been predominately obtained from 3D polymer templates manufactured by direct laser writing which were subsequently inverted using high index materials. The incorporation of high index materials enhances the reflectivity of a given 3D structure considerably, but it inevitably increases the complexity of the fabrication process. Here we demonstrate the successful fabrication of one-dimensional photonic crystals by 3D direct laser writing using only a single polymer to obtain reflectance values approaching that of a gold reference in the near-infrared spectral range. The necessary periodic variation of refractive index is achieved by utilizing partially filled layers wherein integrated sub-wavelength-sized pillars are utilized as a scaffold while simultaneously providing index contrast to that of solid polymer layers. Bruggemann effective medium theory and simulated reflectivity profiles were then used to optimize the photonic crystals’ design to operate at a desired wavelength of 1.55 μm. After fabrication, the structures of the photonic crystals were compared to the nominal geometry via inspection of SEM micrographs and showed true-to-form fabrication results. A good agreement between the model-calculated and measured FTIR reflection and transmission data is observed demonstrating the ease of predictive design with this method.

Si NP structures have a great potential for thermoelectric and cooling device applications. However, current fabrication techniques are too complicated. Furthermore, it is difficult to modulate the properties of the NP by those methods. In this work, we proposed an excellent method to fabricate the NP structure. The water-repellent characteristic of the fabricated Si nanopillar (NP) structure was investigated, and we try clearing the contact angle of density dependence for Si-NPs structure.

12 nm in diameter Si-NPs structure with various density ranging from 1.6 x10¹¹/cm² (low-density) to 7.1 x10¹¹ /cm² (high-density) were fabricated. These samples were fabricated with a unique technique of a bio-template mask and a neutral beam etching. The bio-template mask is a protein shell with an iron oxide core, called ferritin. The density of the Si-NPs can be easily adjusted by modulating the distance between ferritins. The ferritin arrangement was carefully adjusted by controlling the length of the decorated poly( ethylene - glycol ) (PEG); a spin-coating was carried out for this arrangement process . Thereafter, an etching process was done by a neutral beam etching (NBE) technique . The NBE process could realize the damage - less etching on the surface/interface utilizing a bottom electrode that neutralize s ion in pulsed-plasma. The NBE process could realize the damage-less etching on the surface/interface utilizing a bottom electrode that neutralizes ion in pulsed-plasma. NBE can also minimize a UV irradiation to the sample which is beneficial to reduce the occuring lattice defects.

For many years integrated circuits and microstructure devices fabrication, getting smaller features in order to achieve higher density has been a challenge with lithography and dry plasma etching technology improvements. Now, the fabrication of new microsystems need the introduction of materials (like Pt, Ni, Mn, Fe, Co, etc …) incompatible with stand plasma etching. Leading to new stringent etch requirement, the Ion Beam Etching Process enables to address these technological issues.

In this paper, the ion beam milling process is not presented as a simple technological step after lithography and before stripping steps. We will discuss about a complete technological step including lithography, etching and stripping. All the applications presented need metallic compounds or metallic alloys stack etching for microsystem applications. We will presented an empirical model based on experiments to describe fences formation during ion beam etching.

Different design with squared and rounded patterns were tested. Isolated and array of patterns were processed to simulate the impact of design and the role of etching surface in fences formation. All the design were also used for differents metallic stacks and metal simple layer in order to understand the impact of material in fences formation.

Presently, it is an enormous challenge to react organic surfaces with a molecular-level of control, and this largely because current methods, including UV-ozone and electron irradiation, are inherently crude. Unsurprisingly, the precedence of reactions on organic (molecular) surfaces is minimal as these materials do not have the long rich history of inorganic surfaces. As a consequence, the factors defining reactivity have not been effectively evaluated. In contrast with inorganic surfaces which are strongly held together by metallic or covalent bonding, organic surfaces are only held together by relatively weak van der Waals interactions. Furthermore, reactions on organic surfaces are highly anisotropic, where molecular orientation within the lattice is cornerstone to the reactivity of a surface. We examine these and other factors by exposing acene thin films to reactive vapors under a controlled parameter space. Reaction extent was determined using polarization modulation infrared reflection absorption spectroscopy and x-ray photoelectron spectroscopy, and surface morphology was evaluated using atomic force microscopy and scanning electron microscopy. We show that with only mild thermal activation, a surface reaction extended into the subsurface, which we attribute to the weak intermolecular interactions comprising the surface. Classical reaction models were evaluated for their predictive ability and it was found that surface reactions most closely resembled solution/gas phase precedence, while diffusion into the surface plays a small role. Finally, reactivity on organic surfaces are profoundly influenced by defects in a manner not that dissimilar to that of their inorganic counterparts. This work demonstrates the important factors guiding reactivity on organic surfaces, and offers insights into controlling chemical functionalization on the molecular-level.

The TESLA JT SPM provides access to more than 5 days SPM measurement time at temperatures down to 1K (⁴He operation) with magnetic fields larger than B > 3T. Careful thermal design of the bath cryostat and JT cooling stage as well as the integrated UHV magnet lead to exceptionally low LHe consumption of only 11 liters LHe for 120 hours, specifically also during magnet operation and field variation. The external JT Helium supply allows for ³He operation and significantly lower temperatures in the range of 500mK.

The microscope head is a proven, highly stable design developed specifically for high magnetic field environments. It offers the full range of SPM measurements modes, including Scienta Omicron’s leading QPlus AFM technology.

Safe and independent tip/sample exchange under optical control is one of several key ease-of-use features delivering dependable high performance SPM and successful scientific work.

In contrast to a conventional wet magnet concept, the dry split-pair magnet provides for optical access enabling various optical experiments and even in-situ evaporation into the SPM at low temperatures.

We will discuss the technical concept and will show performance evaluation measurements at T=1K that prove stability below 1pm as well as energy resolution on superconductors.

The device potential of multicomponent films in various electronic, magnetic and optical applications critically depends on (i) the chemical composition and (ii) the kinetic energy of the atomic species arriving at the film growth surface. We present two novel methods and instrumentation that allow analysis and control of both composition and energy spectrum of the deposition species. Pulsed Laser Deposition (PLD), a well-known deposition method for multi component materials has been chosen to demonstrate these in-situ diagnostics, providing researchers and engineers an immediate feedback in real-time.

The first tool, called Low Angle X-ray Spectrometer (LAXS), executes quantitative analysis of multiple X-ray spectra emitted by the film-substrate system under the impact of a high-energy electron beam. As the film thickness increases, LAXS follows the evolution of the x-ray spectrum dynamically, and applies special analytical algorithm to find film composition. To validate LAXS, we have chosen multi-elemental compound Y-Ba-Cu-O and demonstrated the efficiency of the technique in identifying the deposition conditions that result in the stoichiometric YBa₂Cu₃ cation composition needed for optimum superconducting properties. In another example using Zn-Ti-Cr continuous compositional spreads, LAXS provides a 2D- map of the resulting compositions that the user can correlate with the distribution of physical properties of interest.

The second tool is the Ion Energy Spectrometer (IES), a differential retarding field energy analyzer, which probes kinetic energy distribution of ions at the growth substrate. Depending on a number of system variables, actual energetics of ions arriving at the growth surface is a critical process parameter that needs careful optimizations. As an example, we analyze the energy spectrum of CeO₂ as a function of oxygen partial pressure in a typical PLD case. Ions of energy as high as >100 eV are present, while majority of the ions are distributed in the 5 - 40 eV range. By varying oxygen background pressure, the IES spectrum is fine-tuned to have a spectral maximum at ≤10 eV- desirable for non-thermally activated, yet soft film growth. The IES also provides several operational modes, including quick acquisition of Time-Of-Flight spectrum.

The eyes of moths own a feature of unique significance that they reflect little or no light. The dome-like patterns in a depth of approximately 200 nm with pitches of about 200 nm function as a surface with graded refractive index to reduce the reflections.

In recent years, the booming of large screen TVs and smart phones brings increasing attentions for the AR moth-eye structures in sub-wavelength for panels. The AR moth-eye structures applied on smart phone glass displays require finer high resolution and well-oriented patterns, as well as much higher ability to sustain finger frictions and environmental contamination. However, the ideal moth-eye like structure is acknowledged to be parabolically curved domes, which has been rarely systematically demonstrated, and the reported methods suffer from the long-term sustainability. Formed by coating, those reported nanoparticles spheres can be easily peeled off from the surface inevitably by scratching or sticking in either hard or soft pressing from the first steps. The sustainability and reproducibility, thus the reduction of total cost of ownership, are strongly hindered as a consequence. Therefore, such critical issues have not been properly tackled.

An attempt has been made in this work to focus on the sustainability and reproducibility of the moth-eye structures fabricated in profile of parabolically curved domes. A master defining the resolution of the sub-wavelength structures is prepared, commonly by means of EBL. The transfer of large area moth-eye nanostructures is conducted by soft UV-NIL. The next critical step is to etch the substrate for sloping sidewalls, i.e. in an isosceles trapezoid from a cross-sectional view. After that, the silicon substrate is thermally oxidized, in which way the domes can be achieved taking advantage of the variation of oxidation rate at the structure corner, sidewall and bottom. By this step, a template featuring highly ordered moth-eye nanostructures in profile of parabolically curved dome of sub-wavelength resolution is well defined. The moth-eye patterns will be transferred onto the target glass substrate though soft UV-NIL and subsequent processing. The dome structures are made eventually in the substrate via covalent bonding, rather than physical adhesion in case of nanoparticle spheres. Loss of nanoparticles due to pressing, sticking, scratching and so on is hardly an issue.

The moth-eye nanostructures patterned in glass are expected to show improved reflectivity of the incident sunlight in sub-wavelength application of portable electrical devices such as smart phone glass displays. Soft UV-NIL enables its potential for direct large area replications.

Fano resonances (FRs) in a semiconductor 2D nanostructure (NS) geometrically inhomogeneous along the propagation of the electron wave (EW) (the x-axis) are theoretically investigated. As is known, FRs [1] arise from interference of EWs propagating along two channels:one of them in the continuous energy spectrum, and the other - a quasistationary state against the background of this spectrum. The considered symmetric along the z-axis NS consisted of three sequentially arranged rectangular quantum wells (QWs) in which the motion of the particle was limited along the z-axis:QW₁ of width L₁ at x < - a, QW₂ of width L₂ at - a < x < a and QW₃ of width L₁ at x > a. It was assumed that L₁ < L₂. In this case, the potential along the x-axis abruptly changed at the points x = - a and x = a, and in each QW a series of quantum-size subbands (QSSs) was formed. Thus the energy QSSs E_(1),n in QW₁ and E_(3),n in QW₃ was the same for the same width of this QWs. In a wide QW₂ the distance between QSSs E_(2),m was less than the distance similar number of QSSs in QW₁ and QW₃ ( figures 1,2,3 in round brackets indicate the number of the QW, and n and m – number of QSSs, respectively, in QW₁, QW₃ and QW₂). Therefore, in QW₂ formed longitudinal rectangular QW along the x- axis of width 2a, due to the different energy position of the QSSs in QW₁, QW₃ and QW₂. These QWs also formed QSSs E_x(2),t due to the restriction of motion in them along the x-axis. These QSSs were the quasistationary states providing the formation of FRs in the considered NS. We calculated the dependence of the NS transmission coefficient |T(Ex)|² for the electron wave of the unit amplitude propagating from the QW₁ along the lower QSS E_(1),1,on its longitudinal energy E_x in the range E_(1),1< E_x< E_(1),2. The widths of the QWs L₁ and L₂ in the symmetric on the z-axis NS were chosen so that in this range of variation of E_x in QW₂ there exist at least two longitudinal QWs of different depths with quasistationary states: QW_х,1, formed by the QSSs Е_(1),2, Е_(2),2 and E_(3),2, and lying higher in energy QW_х,2, formed by the QSSs E_(1),2, E_(3),2 and E_(3),2. In this case, the wave propagation to QW₂ was possible only through this overlying QSS, since the transition from the QSS E_(1),1 in QW₁ to the Е_(2),2 QSS in QW₂ was forbidden by the symmetry of the NS. At the same time, the wave propagation in the channel in the continuous spectrum and in the quasistationary state with the corresponding energy was possible in QW₂. Further, the interfering waves propagated in QW₃ also along one lower QSS with energy E _{(3),1 =} E_(1),1.Thus, when changing E_x, depending on |T(Ex)|², FRs appeared.

[1] U. Fano, Phys. Rev. 124. 1866 (1961).

Indirect transition and opposite circular polarization of Interlayer Exciton in a MoSe2/WSe2 van der Waals Heterostructure

Hsun-Jen Chuang, A.T. Hanbicki, M.R. Rosenberger, C. Stephen Hellberg, S.V. Sivaram, K.M. McCreary, I.I. Mazin, and B.T. Jonker

Naval Research Laboratory, Washington, DC 20375

An emerging class of heterostructures involves monolayer semiconductors such as many of the transition metal dichalcogenides (TMDs) which can be combined to form van der Waals heterostructures (vdWHs). One unique new heterostructure property is an interlayer exciton (ILE), a spatially indirect, electron-hole pair with the electron in one TMD layer and the hole in the other.

In this report [1], we use state-of-the-art preparation techniques [2] to create MoSe2/WSe2 heterostructures encapsulated in hBN. We observe ILE emission around 1.35 eV at room temperature and resolve this emission into two distinct peaks (ILE1 and ILE2) separated by 24 meV at zero field at 5 K. Furthermore, we demonstrate that the two emission peaks have opposite circular polarizations with up to +20% for the ILE1 and -40% for ILE2 when excited by circularly polarized light. Ab initio calculations provide an explanation of this unique and potentially useful property and indicate that it is a result of the indirect character of both electronic transitions. These peaks are double indirect excitons. i.e. indirect in both real and reciprocal space, split by relativistic effects.

This work was supported by core programs at NRL and the NRL Nanoscience Institute, and by the Air Force Office of Scientific Research #AOARD 14IOA018-134141. This work was also supported in part by a grant of computer time from the DoD High Performance Computing Modernization Program at the U.S. Army Research Laboratory Supercomputing Resource Center.

[1] Hanbicki, Aubrey T., Hsun-Jen Chuang, et al. "Double Indirect Interlayer Exciton in a MoSe2/WSe2 van der Waals Heterostructure." ACS Nano 12 (5) 2018: 4719-4726.

[2] Matthew R Rosenberger, Hsun-Jen Chuang, et al. “Nano-“Squeegee” for the Creation of Clean 2D Material Interfaces” ACS applied materials & interfaces 10 (12) 2018: 10379-10387

Materials science is undergoing profound changes, driven by continual improvements to instrumentation that have resulted in an explosion in the data volume, dimensionality, complexity, and variety, in addition to increased accessibility to high-performance computing (HPC) resources, and more sophisticated computer algorithms than ever before. However, the software supplied with the instruments such as microscopes typically do not provide access to advanced data analysis routines. In addition, such software store measurement data in proprietary file formats and are very expensive to license. These proprietary software and file formats not only impede data analysis but also hinder continued research and instrument development, especially in the era of “big data”. Therefore, moving to the forefront of data-intensive materials research requires general and unified data curation and analysis platforms that are HPC-ready and open source.

We have developed the Universal Spectroscopy and Imaging Data (USID) model, which is an instrument-agnostic data schema capable of representing data of any size, dimensionality, or complexity acquired on a regular grid of positions or random positions as in compressed sensing. This USID schema is stored in standardized hierarchical data format files (HDF5) that can be manipulated using any programming language, scale well from kilobyte to terabyte sized datasets. Consequently, USID files are curation-ready and therefore both meet the guidelines for data sharing issued to federally funded agencies and satisfy the implementation of digital data management as outlined by the United States Department of Energy. The generalized data representation allows data processing to be generalized to a single version of the algorithm regardless of the instruments or even modalities.

We have developed a free and open-source python package called Pycroscopy for analyzing, visualizing and storing data in USID HDF5 files. Our instrument-independent data format has also greatly simplified the correlation of data acquired from multiple instruments, necessary for comprehensive studies of materials. Unlike many other open-source packages that focus on analytical or processing routines specific to an instrument, USID can be readily adopted for different techniques within and beyond microscopy. Furthermore, the generality of Pycroscopy provides material scientists access to a vast and growing library of community-driven data processing and analysis routines that are desperately needed in the age of big data. In summary, Pycroscopy can greatly accelerate materials research and discovery through the realms of big, deep, and smart data.

In this study, auto-dispersing nanospheres with diameters of 20 nm were successfully fabricated via molar mass control and self-assembly of cellulose molecules in ionic liquids. We showed how to prepare cellulose nanoparticles with extremely decreased size and improved dispersion in water. Without extra treatment, the particles could be stably dispersed in the aqueous media longer than a month due to their positive charge on nanoparticle surfaces. The comprehensive analysis demonstrated that the molecular mobility and imidazolium at the reducing ends simultaneously played important roles in increasing the crystallinity and the size uniformity.