Thin tellurium (Te) has been predicted as a potential two dimensional system exhibiting superior thermoelectric and electrical properties. Here, we report the synthesis of high quality ultrathin Te nanostructures and the study of their electrical properties at room temperature. High quality ultrathin Te nanostructures are obtained by high temperature vapor phase deposition on c-plane sapphire substrates. The obtained nanostructures are as thin as 3 nm and exhibit α-Te phase with trigonal crystal structure. Room temperature electrical measurements show significantly higher electrical conductivity compared to prior reports of Te in bulk form or in nanostructure form synthesized by low temperature vapor deposition or wet chemical methods. Additionally, these nanostructures exhibit high field effect hole mobility comparable to black-phosphorous measured previously under similar conditions.
Kazanowska, Barbara K.; Leung, Benjamin L.; Tsai, Miao-Chan T.; Sapkota, Keshab R.; Allerman, Andrew A.; Gunning, Brendan P.; Jones, Kevin S.; Wang, George T.
Voltage-controlled room temperature isothermal reversible spin crossover switching of [Fe{H 2 B(pz) 2 } 2 (bipy)] thin films is demonstrated. This isothermal switching is evident in thin film bilayer structures where the molecular spin crossover film is adjacent to a molecular ferroelectric. The adjacent molecular ferroelectric, either polyvinylidene fluoride hexafluoropropylene or croconic acid (C 5 H 2 O 5 ), appears to lock the spin crossover [Fe{H 2 B(pz) 2 } 2 (bipy)] molecular complex largely in the low or high spin state depending on the direction of ferroelectric polarization. In both a planar two terminal diode structure and a transistor structure, the voltage controlled isothermal reversible spin crossover switching of [Fe{H 2 B(pz) 2 } 2 (bipy)] is accompanied by a resistance change and is seen to be nonvolatile, i.e., retained in the absence of an applied electric field. The result appears general, as the voltage controlled nonvolatile switching can be made to work with two different molecular ferroelectrics: croconic acid and polyvinylidene fluoride hexafluoropropylene.
Quantum-size-controlled photoelectrochemical (QSC-PEC) etching, which uses quantum confinement effects to control size, can potentially enable the fabrication of epitaxial quantum nanostructures with unprecedented accuracy and precision across a wide range of materials systems. However, many open questions remain about this new technique, including its limitations and broader applicability. In this project, using an integrated experimental and theoretical modeling approach, we pursue a greater understanding of the time-dependent QSC-PEC etch process and to uncover the underlying mechanisms that determine its ultimate accuracy and precision. We also seek to broaden our understanding of the scope of its ultimate applicability in emerging nanostructures and nanodevices.
Golam Sarwar, A.T.M.; Leung, Benjamin; Wang, George T.; Myers, Roberto C.
By using multiple growth steps that separate the nucleation and growth processes, we show that nearly intrinsic InN single nanocrystals of high optical quality can be formed on patterned GaN nanowire arrays by molecular beam epitaxy. The InN nanostructures form into well-defined hexagonal prisms with pyramidal tops. Micro-photoluminescence (μ-PL) is carried out at low temperature (LT: 28.2 K) and room temperature (RT: 285 K) to gauge the relative material quality of the InN nanostructures. Nanopyramidal prisms grown using a three-step growth method are found to show superior quantum efficiency. Excitation and temperature dependent μ-PL demonstrates the very high quality and nearly intrinsic nature of the ordered InN nanostructure arrays.
The goal of this LDRD is to develop a quantum nanophotonics capability that will allow practical control over electron (hole) and photon confinement in more than one dimension. We plan to use quantum dots (QDs) to control electrons, and photonic crystals to control photons. InGaN QDs will be fabricated using quantum size control processes, and methods will be developed to add epitaxial layers for hole injection and surface passivation. We will also explore photonic crystal nanofabrication techniques using both additive and subtractive fabrication processes, which can tailor photonic crystal properties. These two efforts will be combined by incorporating the QDs into photonic crystal surface emitting lasers (PCSELs). Modeling will be performed using finite-different time-domain and gain analysis to optimize QD-PCSEL designs that balance laser performance with the ability to nano-fabricate structures. Finally, we will develop design rules for QD-PCSEL architectures, to understand their performance possibilities and limits.