Session E16: Transport in Nanostructures -- Nanoscale Transport I

Nanostructured gold thermocouple for photodetection

The Seebeck coefficient of a metal depends on the energy- dependent electrical conductivity, which in turn depends on the energy- dependent electron mean free path and the material band structure. At the nanoscale, when the geometric size is comparable with mean free path of the electrons, single metal thermocouples can be fabricated by changing the material geometry across the thermocouple. By using plasmonically-resonant structures, different device geometries and sizes can be used for wavelength sensitive light detection. We will present preliminary experimental data and simulations of single metal gold nanostructures with different geometries that are plasmonically active with IR laser illumination. We will discuss how these devices can be used for photodetection and discuss future applications for these measurements.   

Shot-Noise Measurements of Waveguides with Attractive Electron-Electron Interactions

Mesoscopic devices created at oxide interfaces, such as the LaAlO₃/SrTiO₃ heterostructure [1], exhibit a wide range of exotic behavior that is linked to a strong electron-electron interaction. Electron pairing without superconductivity [2] involves transport of charge-2e carriers and higher-order “Pascal” phases with charge ne (n>2), have been identified in quantum transport measurements. One way to gain further insight to these phases would be to verify the existence of quantized charge by measuring shot noise. Here we describe experimental efforts to reveal shot-noise characteristics in electron waveguide devices whose electronic phases that can be tuned between paired and unpaired state via tuning of magnetic field and chemical potential. These shot noise measurements have the potential to confirm the charge of these exotic electron liquid phases and provide insight into their phase diagrams.

[1] Y.-Y. Pai, A. Tylan-Tyler, P. Irvin, and J. Levy, Rep Prog Phys 81, 036503 (2018).
[2] G. Cheng et al., Nature 521, 196 (2015).   

Observation of 450 GHz surface acoustic waves in suspended polycrystalline films by use of time-resolved resonant soft X-ray scattering

We have used ultrafast optical pumping to generate nanoscale surface acoustic waves (SAWs) from 200 to 450 GHz, 10-50 nm wavelengths, in metallic films on SiN membranes. Our measurement demonstrates a novel soft X-ray elastic scattering mechanism that probes the coupling of the electronic and phononic degrees of freedom on a [Co₉₀Fe₁₀(0.6 nm)/Ni(0.2 nm)]x50 multilayer. The samples have an average grain size of 30 nm and a rms roughness of 1 nm. This nanostructured surface topography allows optical coupling to the in-plane SAW. In a transmission soft X-ray scattering geometry, with circular polarized X-rays tuned to the Ni L₃edge, we observe a prominent charge scatter ring with a radius of approximately 0.2 nm^-1. After optical pumping with fluences ranging from 24-27 mJ/cm², SAWs appeared as ripples on the charge scatter ring that oscillate at ps timescales. We use Brillouin light scattering (BLS) and modeling to identify the SAW as a dilation mode with an in-plane velocity of 5.6 km/s. Surprisingly, the SAW lifetime peaks sharply at q = 0.25 nm^-1(300 GHz), with a value greater than 60 ps, suggestive of a minimum in the SAW correlation length in the case of 2-d localization.   

Phonon Localization in Heat Conduction

The departure from diffusive phonon thermal transport has been extensively observed via a reduction in thermal conductivity in nanostructures. Such non-diffusive behavior has been largely explained with classical size effects, ignoring the wave nature of phonons. Here, we report localization behavior in phonon heat conduction due to multiple scattering and interference of broadband phonon waves, observed through measurements of the thermal conductivities of GaAs/AlAs superlattices with ErAs nanodots randomly distributed at the interfaces. Near room temperature, the measured thermal conductivities increased with increasing number of superlattice periods and eventually saturated, indicating a transition from ballistic to diffusive transport. At low temperatures, the thermal conductivities of the samples with ErAs dots first increased and then decreased with an increasing number of periods, signaling phonon wave localization. This Anderson localization behavior is also validated via atomistic Green’s function simulations. The observation of phonon localization in heat conduction is surprising due to the broadband nature of thermal transport. This discovery suggests a new path forward for engineering phonon thermal transport.   

Optimization of Electron-Phonon Coupling and Electronic Transport in Semiconductor Superlattices

Efficient thermoelectric (TE) materials require to maintain low thermal conductivity and high electronic transport properties to attain the phonon-glass-electron-crystal regime. Phonon engineering approaches in superlattices (SL) have been demonstrated to significantly hinder thermal conductivity. However, investigation of electron-phonon interaction (EPI) is a key factor to design devices with high electronic performance. In this work, we investigate electron-phonon coupling and electronic transport in Si/Ge SLs by employing first principle DFT calculations in conjunction with semi-classical Boltzmann transport theory. Computation of EPI requires fine sampling of Brillouin zone and, therefore, becomes expensive even for small systems. In order to reduce computational cost, we employ an assumption that the scattering rates are proportional to the electronic density of states (DOS). [1] We establish that the constants of proportionality linearly depend on SL period, composition and strain by rigorous computations. This relationship allows us to predict EPI rates for arbitrary Si/Ge SL and to determine the parameters to optimize electronic transport and therefore, thermoelectric performance of short period SL.
[1] JAP 122, no. 17 175102 (2017).   

Controlling hot-electron thermalization in nanoscale materials

Non-equilibrium energy transfer between hot electrons and phonons plays an important role in the design and operation of photovoltaic and nanoelectronic devices. In this talk, I will show how symmetries in low dimensional materials can impact electron-phonon coupling and the timescale of “hot” electron thermalization. Using a recently-developed first-principles Boltzmann transport equation framework accounting for electron-phonon and phonon-phonon interactions [1], I will show this effect can be used to control hot electron dynamics and phonon bottlenecks for experimentally-synthesized low-dimensional devices. In particular, I will show how such non-equilibrium dynamics can be controlled by external gate potentials.
[1] Sadasivam, Chan, Darancet PRL 119, 136602 (2017)
  

Machine Learning Electronic Transport Properties of Multilayered Semiconductor Nanostructures

Computing components are being aggressively inserted into semiconductor architectures to perform operations at high rates, for a broad range of applications. The contact interfaces between these components dictate performance, especially as device dimension approaches nanoscale. Ab initio methods become expensive and infeasible to predict electronic properties of systems with large number of configurational degrees of freedom. In this study, we employ machine learning (ML) algorithms to predict electronic structure and transport of non-ideally fabricated multilayered thin film Si/Ge nanostructures. The algorithm is trained on inexpensive ~200 DFT calculations of SixGe1-x substitutional alloys, by exploiting the relationship between local atomic environments and electronic properties. The predictor variables are obtained with Voronoi tessellation approach and the response variables are calculated with decision tree regression algorithm. Our model has shown remarkable ability to predict band structures and Onsager transport coefficients of large non-ideal superlattices. The ML framework will facilitate the development of inverse design approach to engineer interface profiles for desired performance of integrated semiconductor architectures.   

Ballistic length scale of heat transport in the subwavelength limit

We present a comprehensive study of phonon ballistic length scale using a Casimir--Rayleigh model of heat transport. This model incorporates temperature-dependent Casimir radiation and wavelength-dependent Rayleigh scattering. We exploit this definition to understand heat transport in newly synthesized silicon metalattices, which consist of a finely controlled, three-dimensional arrangement of nanometer-sized cavities in crystalline silicon. Through computational simulation and experimental validation, we show that the heat conductivity of metalattices exhibits a minimum as a function of the cavity diameter at constant porosity. This departure from Casimir's linear scaling law can be understood in terms of the geometry dependence of the phonon mean free path in a network of cavities.   

Specular Reflection Leads to Maximum Reduction in Thermal Phonon Conductivity

A material thermal conductivity is not a fixed physical property but it can be controlled by modifying the transport properties of thermal phonons. In recent years, a large number of experiments have been reported where phonon mean free paths in nanostructures are reduced by orders of magnitude. In contrast to established work that use the diffuse surface scattering of phonons as the physical mechanism to reduce the thermal conductivities, in this talk we show that the largest reduction of thin film heat conduction is achieved via specular surface scattering. Our results create new opportunities for heat conduction manipulation since smooth surfaces – in contrast to rough surfaces – can be more effective on suppressing thin film phonon heat conduction.   

Coupling of Boron Dipyrromethene Dye Excitons to Plasmonic Surface Lattice Resonances in Aluminum Nanodisk Arrays

When plasmonic metal nanoparticles are arranged in extended, one- or two-dimensional periodic arrays, the localized surface plasmon resonances (LSPRs) of the individual particles will couple radiatively to form a collective, propagating photonic-plasmonic mode known as a surface lattice resonance (SLR). Currently, SLRs and their potential applications in photonic devices, such as solar cells and light-emitting diodes, are growing topics of interest in the literature. In particular, the interaction of propagating, delocalized SLRs with the highly localized excitons of organic dye molecules is being investigated, and exotic phenomena such as the Bose-Einstein condensation of polaritons composed of dye excitons coupled to SLRs was recently reported(1). We report on the fabrication of SLR-supporting arrays of aluminum nanodisks on glass, and the coupling of these SLRs to dye excitons via coating of the arrays with dye-doped poly(methyl methacrylate). In particular, the interaction of the SLRs with boron dipyrromethene (BODIPY) dyes is examined, and the resulting effects, including angle-dependent fluorescent emission and enhancement of energy transfer between two different BODIPY dyes, are reported.

(1) Hakala et al. Nature Physics 2018, 14 (7), 739.
  

Terahertz Spectroscopy of Metallic Single-Wall Carbon Nanotubes

Optically generated, Coulombically bound electron-hole pairs, known as excitons, are rarely observed in metals due to strong electrostatic screening. However, in quantum-confined systems, such as one-dimensional (1D) single-wall carbon nanotubes (SWCNTs), screening effects are suppressed giving rise to exciton-dominated optical spectra in both semiconducting and metallic SWCNTs. Because of the difficulty of creating highly isolated 1D metallic environments, these metallic excitons are poorly studied. Here, we use terahertz absorption of single-chirality enriched SWCNTs at low temperatures to examine collective phenomena in 1D. We prepared, single-chirality (5,5) metallic and enriched (6,5) semiconducting SWCNTs in high-purity using aqueous two-phase extraction and characterized these fractions by optical methods. Enriched SWCNTs were immersed in a broadly transparent polymer matrix which preserved SWCNT individualization for cryogenic measurement. Using terahertz time-domain spectroscopy, we observed low-frequency plasmon absorption in metallic SWCNTs across a broad temperature range. This work provides the foundation for in-depth study of excitonic and plasmonic phenomena in single-chirality SWCNTs.
  

Fabrication and characterization of nanoscale devices using Local anodic oxidation technique

Graphene is one of the most promising materials as a flexible transparent electrode because of its high transparency and ultrahigh carrier mobility. Up to now, there are a number of methods to pattern graphene for application as electrodes. A commonly used patterning method is a combination of lithography and plasma etching, which causes undesired graphene defects during the etching process. Attempts to develop a patterning method that can simplify the fabrication process with minimal degradation is still a challenge.
Here, we applied an electrochemical process based on Atomic force microscopy (AFM), Local anodic oxidation (LAO) technique, to define graphene electrodes with minimized graphene defect by eliminating the etching process. In addition, it is easy to manufacture a short channel device because the AFM tip has a nanometer order radius.
This technique paves the way for the fabrication of organic-based devices where it is difficult to fabricate with short channels.
  

Electron, phonon, and interfacial transport in 2D materials and heterostructures

This talk will present recent highlights from our research on two-dimensional (2D) materials. Results span from fundamental measurements and simulations, to applications taking advantage of unusual 2D material properties. We measured record velocity saturation in graphene [1], and the thermal properties of graphene nanoribbons [2]. We have also grown monolayer 2D semiconductors by CVD over large areas, including MoS₂ [3], WSe₂, MoSe₂ [4], and multilayer MoTe₂ and WTe₂ [5]. ZrSe₂ and HfSe₂ have native high-K dielectrics ZrO₂ and HfO₂, which are of key technological relevance [6]. Improving electrical contacts [7], we demonstrated 10 nm transistors using monolayer MoS₂, with the highest current reported to date (>400 µA/µm), near ballistic limits [8]. Current density in such 2D devices is ultimately limited by self-heating and phonon scattering [9], in part due to the weak van der Waals bonds between 2D materials and their environment, which lead to a large thermal resistance of this interface [10]. On the other hand, we exploited this weak interface to improve energy efficiency in phase-change memory [11], and we tuned it by Li intercalation, demonstrating MoS₂-based thermal transistors [12]. These studies reveal fundamental limits and some applications of 2D materials, taking advantage of their unique properties.

[1] M. Yamoah, et al., ACS Nano 11, 9914 (2017). [2] M.-H. Bae et al., Nature Comm. 4, 1734 (2013). [3] K. Smithe et al., ACS Nano 11, 8456 (2017). [4] K. Smithe et al., ACS AMI 1, 572 (2018). [5] M. Mleczko et al., ACS Nano 10, 7507 (2016). [6] M. Mleczko et al., Science Adv. 3, e1700481 (2017). [7] C. English et al., Nano Lett. 16, 3824 (2016). [8] C. English et al., IEEE Intl. Electron Devices Meeting (IEDM), Dec 2016. [9] K. Smithe et al., Nano Lett. 18, 4516 (2018). [10] E. Yalon et al., Nano Lett. 17, 3429 (2017). [11] C. Ahn et al., Nano Lett. 15, 6809 (2015). [12] A. Sood et al. Nature Comm. 9, 4510 (2018).