| Siegfried, Peter E.; Bhandari, Hari; Qi, Jeanie; Ghimire, Rojila; Joshi, Jayadeep; Messegee, Zachary T.; Beeson, Willie B.; Liu, Kai; Ghimire, Madhav Prasad; Dang, Yanliu; Zhang, Huairuo; Davydov, Albert V.; Tan, Xiaoyan; Vora, Patrick M.; Mazin, Igor I.; Ghimire, Nirmal J.: CoTe2: A Quantum Critical Dirac Metal with Strong Spin Fluctuations. In: Advanced Materials, vol. 35, no. 21, pp. 2300640, 2023. (Type: Journal Article | | | )|
Abstract Quantum critical points separating weak ferromagnetic and paramagnetic phases trigger many novel phenomena. Dynamical spin fluctuations not only suppress the long-range order, but can also lead to unusual transport and even superconductivity. Combining quantum criticality with topological electronic properties presents a rare and unique opportunity. Here, by means of ab initio calculations and magnetic, thermal, and transport measurements, it is shown that the orthorhombic CoTe2 is close to ferromagnetism, which appears suppressed by spin fluctuations. Calculations and transport measurements reveal nodal Dirac lines, making it a rare combination of proximity to quantum criticality and Dirac topology.
| Khan, Asir Intisar; Yu, Heshan; Zhang, Huairuo; Goggin, John R.; Kwon, Heungdong; Wu, Xiangjin; Perez, Christopher; Neilson, Kathryn M.; Asheghi, Mehdi; Goodson, Kenneth E; Vora, Patrick M.; Davydov, Albert; Takeuchi, Ichiro; Pop, Eric: Energy Efficient Neuro-Inspired Phase–Change Memory Based on Ge4Sb6Te7 as a Novel Epitaxial Nanocomposite. In: Advanced Materials, vol. 35, no. 30, pp. 2300107, 2023. (Type: Journal Article | | | )|
Abstract Phase-change memory (PCM) is a promising candidate for neuro-inspired, data-intensive artificial intelligence applications, which relies on the physical attributes of PCM materials including gradual change of resistance states and multilevel operation with low resistance drift. However, achieving these attributes simultaneously remains a fundamental challenge for PCM materials such as Ge2Sb2Te5, the most commonly used material. Here bi-directional gradual resistance changes with ≈10× resistance window using low energy pulses are demonstrated in nanoscale PCM devices based on Ge4Sb6Te7, a new phase-change nanocomposite material . These devices show 13 resistance levels with low resistance drift for the first 8 levels, a resistance on/off ratio of ≈1000, and low variability. These attributes are enabled by the unique microstructural and electro-thermal properties of Ge4Sb6Te7, a nanocomposite consisting of epitaxial SbTe nanoclusters within the Ge–Sb–Te matrix, and a higher crystallization but lower melting temperature than Ge2Sb2Te5. These results advance the pathway toward energy-efficient analog computing using PCM.
| Ye, Kevin; Koocher, Nathan Z.; Filippone, Stephen; Niu, Shanyuan; Zhao, Boyang; Yeung, Matthew; Bone, Sharon; Robinson, Adam J.; Vora, Patrick; Schleife, André; Ju, Long; Boubnov, Alexey; Rondinelli, James M.; Ravichandran, Jayakanth; Jaramillo, R.: Low-energy electronic structure of perovskite and Ruddlesden-Popper semiconductors in the Ba-Zr-S system probed by bond-selective polarized x-ray absorption spectroscopy, infrared reflectivity, and Raman scattering. In: Phys. Rev. B, vol. 105, iss. 19, pp. 195203, 2022. (Type: Journal Article | | )|
| Hughes, Ciaran; Finke, Doug; German, Dan-Adrian; Merzbacher, Celia; Vora, Patrick M.; Lewandowski, H. J.: Assessing the Needs of the Quantum Industry. In: IEEE Transactions on Education, pp. 1-10, 2022. (Type: Journal Article | | )|
| Joshi, Jaydeep; Scharf, Benedikt; Mazin, Igor; Krylyuk, Sergiy; Campbell, Daniel J.; Paglione, Johnpierre; Davydov, Albert; Žutić, Igor; Vora, Patrick M.: Charge Density Wave Activated Excitons in TiSe$_2$-MoSe$_2$ Heterostructures. In: APL Materials, vol. 10, no. 1, pp. 011103, 2022. (Type: Journal Article | | )|
| Chowdhury, Sugata; Rigosi, Albert F.; Hill, Heather M.; Vora, Patrick; Walker, Angela R. Hight; Tavazza, Francesca: Computational Methods for Charge Density Waves in 2D Materials. In: Nanomaterials, vol. 12, no. 3, 2022, ISSN: 2079-4991. (Type: Journal Article | | | )|
Two-dimensional (2D) materials that exhibit charge density waves (CDWs)—spontaneous reorganization of their electrons into a periodic modulation—have generated many research endeavors in the hopes of employing their exotic properties for various quantum-based technologies. Early investigations surrounding CDWs were mostly focused on bulk materials. However, applications for quantum devices require few-layer materials to fully utilize the emergent phenomena. The CDW field has greatly expanded over the decades, warranting a focus on the computational efforts surrounding them specifically in 2D materials. In this review, we cover ground in the following relevant theory-driven subtopics for TaS2 and TaSe2: summary of general computational techniques and methods, resulting atomic structures, the effect of electron–phonon interaction of the Raman scattering modes, the effects of confinement and dimensionality on the CDW, and we end with a future outlook. Through understanding how the computational methods have enabled incredible advancements in quantum materials, one may anticipate the ever-expanding directions available for continued pursuit as the field brings us through the 21st century.
| Milas, Peker; Mathab, Sheikh; Abraham, John Bishoy Sam; Alam, Jahangir; Chandrashekar, M. V. S.; Robinson, Adam J.; Vora, Patrick M.; Ozturk, Birol; Spencer, Michael G.: Electronic and optical characterization of bulk single crystals of cubic boron nitride (cBN). In: AIP Advances, vol. 12, no. 9, pp. 095303, 2022, ISSN: 2158-3226. (Type: Journal Article | | | )|
Cubic boron nitride (cBN) is a relatively less studied wide bandgap semiconductor despite its many promising mechanical, thermal, and electronic properties. We report on the electronic, structural, and optical characterization of commercial cBN crystal platelets. Temperature dependent transport measurements revealed the charge limited diode behavior of the cBN crystals. The equilibrium Fermi level was determined to be 0.47 eV below the conduction band, and the electron conduction was identified as n-type. Unirradiated dark and amber colored cBN crystals displayed broad photoluminescence emission peaks centered around different wavelengths. RC series zero phonon line defect emission peaks were observed at room temperature from the electron beam irradiated and oxygen ion implanted cBN crystals, making this material a promising candidate for high power microwave devices, next generation power electronics, and future quantum sensing applications.
| Melis, Scott; Hung, Samantha; Bagade, Chaitali; Chung, Yuri; Hughes, Eleni; Zhang, Xinran; Barbara, Paola; Han, Peize; Li, Tingting; McCusker, Daniel; Hartsmith, Robert; Bertke, Jeffery; Dev, Pratibha; Stone, Iris; Joshi, Jaydeep; Vora, Patrick; Keuren, Edward Van: Charge Transport through Superexchange in Phenothiazine–7,7,8,8-Tetracyanoquinodimethane (PTZ–TCNQ) Cocrystal Microribbon FETs Grown Using Evaporative Alignment. In: ACS Applied Electronic Materials, vol. 4, no. 12, pp. 5973-5983, 2022. (Type: Journal Article | | )|
| Oliver, Sean M; Young, Joshua; Krylyuk, Sergiy; Reinecke, Thomas L; Davydov, Albert V; Vora, Patrick M: Valley phenomena in the candidate phase change material WSe2(1-x)Te2x. In: Communications Physics, vol. 3, no. 1, pp. 10, 2020, ISSN: 2399-3650. (Type: Journal Article | | | )|
Alloyed transition metal dichalcogenides provide an opportunity for coupling band engineering with valleytronic phenomena in an atomically-thin platform. However, valley properties in alloys remain largely unexplored. We investigate the valley degree of freedom in monolayer alloys of the phase change candidate material WSe$_2(1-x)$Te$_2x$. Low temperature Raman measurements track the alloy-induced transition from the semiconducting 1H phase of WSe$_2$ to the semimetallic 1T$_d$ phase of WTe$_2$. We correlate these observations with density functional theory calculations and identify new Raman modes from W-Te vibrations in the 1H alloy phase. Photoluminescence measurements show ultra-low energy emission features that highlight alloy disorder arising from the large W-Te bond lengths. Interestingly, valley polarization and coherence in alloys survive at high Te compositions and are more robust against temperature than in WSe$_2$. These findings illustrate the persistence of valley properties in alloys with highly dissimilar parent compounds and suggest band engineering can be utilized for valleytronic devices.
| Jo, Seong Soon; Singh, Akshay; Yang, Liqiu; Tiwari, Subodh C; Hong, Sungwook; Krishnamoorthy, Aravind; Sales, Maria Gabriela; Oliver, Sean M; Fox, Joshua; Cavalero, Randal L; Snyder, David W; Vora, Patrick M; McDonnell, Stephen J; Vashishta, Priya; Kalia, Rajiv K; Nakano, Aiichiro; Jaramillo, R: Growth Kinetics and Atomistic Mechanisms of Native Oxidation of ZrSSe and MoS Crystals. In: Nano Lett, vol. 20, no. 12, pp. 8592–8599, 2020, ISSN: 1530-6992. (Type: Journal Article | | | )|
A thorough understanding of native oxides is essential for designing semiconductor devices. Here, we report a study of the rate and mechanisms of spontaneous oxidation of bulk single crystals of ZrSSe alloys and MoS. ZrSSe alloys oxidize rapidly, and the oxidation rate increases with Se content. Oxidation of basal surfaces is initiated by favorable O adsorption and proceeds by a mechanism of Zr-O bond switching, that collapses the van der Waals gaps, and is facilitated by progressive redox transitions of the chalcogen. The rate-limiting process is the formation and out-diffusion of SO. In contrast, MoS basal surfaces are stable due to unfavorable oxygen adsorption. Our results provide insight and quantitative guidance for designing and processing semiconductor devices based on ZrSSe and MoS and identify the atomistic-scale mechanisms of bonding and phase transformations in layered materials with competing anions.
| Haripriya, G R; Heitmann, T W; Yadav, D K; Kaphle, G C; Ghimire, Madhav Prasad; Pradheesh, R; Joshi, J; Vora, P; Sethupathi, K; Sankaranarayanan, V; Nair, H S: Spin reorientation in antiferromagnetic Dy2FeCoO6 double perovskite. In: Journal of Physics: Condensed Matter, vol. 33, no. 2, pp. 025802, 2020. (Type: Journal Article | | | )|
We explored the electronic and magnetic properties of the lanthanide double perovskite Dy2FeCoO6 by combining magnetization, Raman and Mössbauer spectroscopy and neutron diffraction along with density functional theory (DFT) calculations. Our magnetization measurements revealed two magnetic phase transitions in Dy2FeCoO6. First, a paramagnetic to antiferromagnetic transition at T N = 248 K and subsequently, a spin reorientation transition at T SR = 86 K. In addition, a field-induced magnetic phase transition with a critical field of H c ≈ 20 kOe is seen at 2 K. Neutron diffraction data suggested cation-disordered orthorhombic structure for Dy2FeCoO6 in Pnma space group which is supported by Raman scattering results. The magnetic structures ascertained through representational analysis indicate that at T N, a paramagnetic state is transformed to Γ5(Cx, Fy, Az) antiferromagnetic structure while, at T SR, Fe/Co moments undergo a spin reorientation to Γ3(Gx, Ay, Fz). The refined magnetic moment of (Fe/Co) is 1.47(4) μ B at 7 K. The antiferromagnetic structure found experimentally is supported through the DFT calculations which predict an insulating electronic state in Dy2FeCoO6.
| Joshi, Jaydeep; Zhou, Tong; Krylyuk, Sergiy; Davydov, Albert V; Zutic, Igor; Vora, Patrick M: Localized Excitons in NbSe 2 -MoSe 2 Heterostructures. In: ACS Nano, pp. acsnano.0c02803, 2020, ISSN: 1936-0851. (Type: Journal Article | | | )|
Neutral and charged excitons (trions) in atomically-thin materials offer important capabilities for photonics, from ultrafast photodetectors to highly-efficient light-emitting diodes and lasers. Recent studies of van der Waals (vdW) heterostructures comprised of dissimilar monolayer materials have uncovered a wealth of optical phenomena that are predominantly governed by interlayer interactions. Here, we examine the optical properties in NbSe$_2$ - MoSe$_2$ vdW heterostructures, which provide an important model system to study metal-semiconductor interfaces, a common element in optoelectronics. Through low-temperature photoluminescence (PL) microscopy we discover a sharp emission feature, L1, that is localized at the NbSe$_2$-capped regions of MoSe$_2$. L1 is observed at energies below the commonly-studied MoSe$_2$ excitons and trions, and exhibits temperature- and power-dependent PL consistent with exciton localization in a confining potential. Remarkably, L1 is very robust not just in different samples, but also under a variety of fabrication processes. Using first-principles calculations we reveal that the confinement potential required for exciton localization naturally arises from the in-plane band bending due to the changes in the electron affinity between pristine MoSe$_2$ and NbSe$_2$ - MoSe$_2$ heterostructure. We discuss the implications of our studies for atomically-thin optoelectronics devices with atomically-sharp interfaces and tunable electronic structures.
| Fox, Joshua J; Bachu, Saiphaneendra; Cavalero, Randal L; Lavelle, Robert M; Oliver, Sean M; Yee, Sam; Vora, Patrick M; Alem, Nasim; Snyder, David W: Chemical vapor transport synthesis, characterization and compositional tuning of ZrSxSe2−x for optoelectronic applications. In: Journal of Crystal Growth, vol. 542, pp. 125609, 2020, ISSN: 00220248. (Type: Journal Article | | )|
| Klein, William P; Rolczynski, Brian S; Oliver, Sean M; Zadegan, Reza; Buckhout-White, Susan; Ancona, Mario G; Cunningham, Paul D; Melinger, Joseph S; Vora, Patrick M; Kuang, Wan; Medintz, Igor L; Díaz, Sebastián A: DNA Origami Chromophore Scaffold Exploiting HomoFRET Energy Transport to Create Molecular Photonic Wires. In: ACS Applied Nano Materials, vol. 3, no. 4, pp. 3323–3336, 2020, ISSN: 2574-0970. (Type: Journal Article | | )|
| Oliver, Sean M; Fox, Joshua J; Hashemi, Arsalan; Singh, Akshay; Cavalero, Randal L; Yee, Sam; Snyder, David W; Jaramillo, R; Komsa, Hannu-Pekka; Vora, Patrick M: Phonons and excitons in ZrSe 2 –ZrS 2 alloys. In: Journal of Materials Chemistry C, vol. 8, no. 17, pp. 5732–5743, 2020, ISSN: 2050-7526. (Type: Journal Article | | | )|
We combine experimental and theoretical methods to understand the impact of long-range Coulomb interactions on phonons and excitons in ZrS x Se 2−x .
| Joshi, Jaydeep; Hill, Heather M; Chowdhury, Sugata; Malliakas, Christos D; Tavazza, Francesca; Chatterjee, Utpal; Walker, Angela R Hight; Vora, Patrick M: Short-range charge density wave order in <math> <mrow> <mn>2</mn> <mi>H</mi> <mtext>−</mtext> <mi mathvariant="normal">T</mi> <mrow> <mi mathvariant="normal">a</mi> <msub> <mi mathvariant="normal">S</mi> <mn>2</mn> </msub> </mrow> </mrow> </math>. In: Physical Review B, vol. 99, no. 24, pp. 245144, 2019, ISSN: 2469-9950. (Type: Journal Article | | )|
| Vora, Patrick M; Bracker, Allan S; Carter, Samuel G; Kim, Mijin; Kim, Chul Soo; Gammon, Daniel: Strong coupling of a quantum dot molecule to a photonic crystal cavity. In: Physical Review B, vol. 99, no. 16, pp. 165420, 2019, ISSN: 2469-9950. (Type: Journal Article | | )|
| Zhang, Kehao; Wang, Yuanxi; Joshi, Jaydeep; Zhang, Fu; Subramanian, Shruti; Terrones, Mauricio; Vora, Patrick; Crespi, Vincent; Robinson, Joshua A: Probing the origin of lateral heterogeneities in synthetic monolayer molybdenum disulfide. In: 2D Materials, vol. 6, no. 2, pp. 025008, 2019, ISSN: 20531583. (Type: Journal Article | | | )|
Synthetic two-dimensional (2D) materials provide an opportunity to realize large-scale applications in next generation electronic and optoelectronic devices. One of the biggest challenges of synthetic 2D materials is the lateral heterogeneity such as non-uniform strain, composition and defect density. The electronic and optical properties are found to be not uniform in many cases, even within a single crystalline domain, potentially limiting synthetic 2D materials in advanced devices. In this work, we probe the origin of the widely observed lateral heterogeneities in synthetic monolayer MoS2. Epitaxial single crystalline domains (∼10 $mu$m) are optically homogeneous and uniform with 0.3%-0.4% tensile strain, while misoriented domains (>20 $mu$m) exhibit distinct photoluminescence (PL) emissions from the center to the edge, along with released strain at the center. Temperature-dependent Raman and PL mapping reveals that the center of non-epitaxial domains exhibits an enhanced PL due to increased defect density. Density function theory (DFT) calculations suggest that oxygen defects can readily lead the loss of epitaxy, consistent with our observation of a MoOx core-shell structure that only exists in misoriented domains. Combining experiment and DFT, we hypothesize that two growth mechanisms, solid-solid and vapor-solid growth, may be responsible for the lateral heterogeneities.
| Díaz, Sebastián A; Oliver, Sean M; Hastman, David A; Medintz, Igor L; Vora, Patrick M: Increased Transfer Efficiency from Molecular Photonic Wires on Solid Substrates and Cryogenic Conditions. In: Journal of Physical Chemistry Letters, vol. 9, no. 13, pp. 3654–3659, 2018, ISSN: 19487185. (Type: Journal Article | | | )|
Molecular photonic wires (MPWs) are tunable nanophotonic structures capable of capturing and directing light with high transfer efficiencies. DNA-based assembly techniques provide a simple and economical preparation method for MPWs that allows precise positioning of the molecular transfer components. Unfortunately, the longest DNA-based MPWs (∼30 nm) report only modest transfer efficiencies of ∼2% and have not been demonstrated on solid-state platforms. Here, we demonstrate that DNA-based MPWs can be spin-coated in a polymer matrix onto silicon wafers and exhibit a 5-fold increase in photonic transfer efficiency over solution-phase MPWs. Cooling these MPWs to 5 K led to further efficiency increases ranging from ∼40 to 240% depending on the length of the MPW. The improvement of MPW energy transport efficiencies advances prospects for their incorporation in a variety of optoelectronics technologies and makes them an ideal test bed for further exploration of nanoscale energy transfer.
| Zhang, Kehao; Bersch, Brian M; Joshi, Jaydeep; Addou, Rafik; Cormier, Christopher R; Zhang, Chenxi; Xu, Ke; Briggs, Natalie C; Wang, Ke; Subramanian, Shruti; Cho, Kyeongjae; Fullerton-Shirey, Susan; Wallace, Robert M; Vora, Patrick M; Robinson, Joshua A: Tuning the Electronic and Photonic Properties of Monolayer MoS2 via In Situ Rhenium Substitutional Doping. In: Advanced Functional Materials, vol. 28, no. 16, pp. 1706950, 2018, ISSN: 16163028. (Type: Journal Article | | | )|
Doping is a fundamental requirement for tuning and improving the properties of conventional semiconductors. Recent doping studies including niobium (Nb) doping of molybdenum disulfide (MoS2) and tungsten (W) doping of molybdenum diselenide (MoSe2) have suggested that substitutional doping may provide an efficient route to tune the doping type and suppress deep trap levels of 2D materials. To date, the impact of the doping on the structural, electronic, and photonic properties of in situ-doped monolayers remains unanswered due to challenges including strong film substrate charge transfer, and difficulty achieving doping concentrations greater than 0.3 at%. Here, in situ rhenium (Re) doping of synthetic monolayer MoS2 with ≈1 at% Re is demonstrated. To limit substrate film charge transfer, r-plane sapphire is used. Electronic measurements demonstrate that 1 at% Re doping achieves nearly degenerate n-type doping, which agrees with density functional theory calculations. Moreover, low-temperature photoluminescence indicates a significant quench of the defect-bound emission when Re is introduced, which is attributed to the MoO bond and sulfur vacancies passivation and reduction in gap states due to the presence of Re. The work presented here demonstrates that Re doping of MoS2 is a promising route toward electronic and photonic engineering of 2D materials.
| Oliver, Sean M; Beams, Ryan; Krylyuk, Sergiy; Kalish, Irina; Singh, Arunima K; Bruma, Alina; Tavazza, Francesca; Joshi, Jaydeep; Stone, Iris R; Stranick, Stephan J; Davydov, Albert V; Vora, Patrick M: The structural phases and vibrational properties of Mo1-xWxTe2 alloys. In: 2D Materials, vol. 4, no. 4, pp. 045008, 2017, ISSN: 20531583. (Type: Journal Article | | | )|
The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe2 crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T′ semimetallic phase at high temperatures. Alloying MoTe2 with WTe2 reduces the energy barrier between these two phases, while also allowing access to the Td Weyl semimetal phase. The Mo1-x WxTe2 alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessitates a detailed understanding of the phase composition in the MoTe2-WTe2 system. We combine polarization-resolved Raman spectroscopy with x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study bulk Mo1-xWxTe2 alloys over the full compositional range x from 0 to 1. We identify Raman and XRD signatures characteristic of the 2H, 1T′, and Td structural phases that agree with density-functional theory (DFT) calculations, and use them to identify phase fields in the MoTe2-WTe2 system, including single-phase 2H, 1T′, and Td regions, as well as a two-phase 1T′ + Td region. Disorder arising from compositional fluctuations in Mo1-xWxTe2 alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T′-MoTe2 mode and the enhancement of a double-resonance Raman process in 2H-Mo1-x WxTe2 alloys. Compositional fluctuations limit the phonon correlation length, which we estimate by fitting the observed asymmetric Raman lineshapes with a phonon confinement model. These observations reveal the important role of disorder in Mo1-xWxTe2 alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system.
| Gammon, D; Carter, S; Bracker, A S; Vora, P: Method for generating a single photon for quantum information processing. 2017. (Type: Miscellaneous | | )|
| Oliver, Sean M; Fairfield, Jessamyn A; Bellew, Allen T; Lee, Sunghun; Champlain, James G; Ruppalt, Laura B; Boland, John J; Vora, Patrick M: Quantum point contacts and resistive switching in Ni/NiO nanowire junctions. In: Applied Physics Letters, vol. 109, no. 20, pp. 203101, 2016, ISSN: 0003-6951. (Type: Journal Article | | | )|
Metal oxide devices that exhibit resistive switching are leading candidates for non-volatile memory applications due to their potential for fast switching, low-power operation, and high device density. It is widely accepted in many systems that two-state resistive behavior arises from the formation and rupture of conductive filaments spanning the oxide layer. However, means for controlling the filament geometry, which critically influences conduction, have largely been unexamined. Here, we explore the connection between filament geometry and conductance in a model resistive switching system based on the junction of two nickel/nickel oxide core/shell nanowires. Variable temperature current-voltage measurements indicate that either wide metallic filaments or narrow semiconducting filaments can be preferentially formed by varying the current compliance during electroformation. Metallic filaments behave as a conventional metallic resistance in series with a small barrier, while semiconducting filaments behave as quantum point contacts. The ability to tune filament geometry and behavior through the electroforming process may open avenues for enhanced functionality in nanoscale memristive systems.
| Beams, Ryan; Cançado, Luiz Gustavo; Krylyuk, Sergiy; Kalish, Irina; Kalanyan, Berç; Singh, Arunima K; Choudhary, Kamal; Bruma, Alina; Vora, Patrick M; Tavazza, Francesca; Davydov, Albert V; Stranick, Stephan J: Characterization of Few-Layer 1T′ MoTe 2 by Polarization-Resolved Second Harmonic Generation and Raman Scattering. In: ACS Nano, vol. 10, no. 10, pp. 9626–9636, 2016, ISSN: 1936-0851. (Type: Journal Article | | | )|
We study the crystal symmetry of few-layer 1T′ MoTe2 using the polarization dependence of the second harmonic generation (SHG) and Raman scattering. Bulk 1T′ MoTe2 is known to be inversion symmetric; however, we find that the inversion symmetry is broken for finite crystals with even numbers of layers, resulting in strong SHG comparable to other transition-metal dichalcogenides. Group theory analysis of the polarization dependence of the Raman signals allows for the definitive assignment of all the Raman modes in 1T′ MoTe2 and clears up a discrepancy in the literature. The Raman results were also compared with density functional theory simulations and are in excellent agreement with the layer-dependent variations of the Raman modes. The experimental measurements also determine the relationship between the crystal axes and the polarization dependence of the SHG and Raman scattering, which now allows the anisotropy of polarized SHG or Raman signal to independently determine the crystal orientation.
| Joshi, Jaydeep; Stone, Iris R; Beams, Ryan; Krylyuk, Sergiy; Kalish, Irina; Davydov, Albert V; Vora, Patrick M: Phonon anharmonicity in bulk Td-MoTe2. In: Applied Physics Letters, vol. 109, no. 3, pp. 031903, 2016, ISSN: 00036951. (Type: Journal Article | | | )|
We examine anharmonic contributions to the optical phonon modes in bulk Td-MoTe2 through temperature-dependent Raman spectroscopy. At temperatures ranging from 100 K to 200 K, we find that all modes redshift linearly with temperature in agreement with the Grüneisen model. However, below 100 K, we observe nonlinear temperature-dependent frequency shifts in some modes. We demonstrate that this anharmonic behavior is consistent with the decay of an optical phonon into multiple acoustic phonons. Furthermore, the highest frequency Raman modes show large changes in intensity and linewidth near T ≈ 250 K that correlate well with the T d → 1 T ′ structural phase transition. These results suggest that phonon-phonon interactions can dominate anharmonic contributions at low temperatures in bulk Td-MoTe2, an experimental regime that is currently receiving attention in efforts to understand Weyl semimetals.
| Yang, Lily; Carter, Samuel G; Bracker, Allan S; Yakes, Michael K; Kim, Mijin; Kim, Chul Soo; Vora, Patrick M; Gammon, Daniel: Optical spectroscopy of site-controlled quantum dots in a Schottky diode. In: Applied Physics Letters, vol. 108, no. 23, pp. 233102, 2016, ISSN: 0003-6951. (Type: Journal Article | | | )|
Most renewable energy technologies suffer from an intermittent characteristic due to the diurnal and seasonal patterns of the natural resources needed for power generation; therefore, a complementary energy storage system must be considered. The pumped hydropower plant is a suitable alternative to consider as an energy storage device for hybrid systems. The hybrid optimization model for electric renewables (HOMER) optimization model is widely used around the globe for designing, comparing, or evaluating the performance of hybrid power systems, but it does not include an explicit component to model a pumped hydropower facility. This paper describes a method for representing a pumped hydropower plant by creating an equivalent battery in HOMER, and the procedure was accompanied by a detailed example. An additional example of a wind-hydro hybrid power system with controlled parameters is presented to validate the method. The results support that the procedure explained in this paper adequately represents the pumped hydropower plant as an equivalent battery.
| Gammon, Daniel; Carter, Samuel G; Bracker, Allan S; Vora, Patrick M: Single photon source based on a quantum dot molecule in an optical cavity. 2016. (Type: Miscellaneous | | )|
| Vora, Patrick M; Bracker, Allan S; Carter, Samuel G; Sweeney, Timothy M; Kim, Mijin; Kim, Chul Soo; Yang, Lily; Brereton, Peter G; Economou, Sophia E; Gammon, Daniel: Spin–cavity interactions between a quantum dot molecule and a photonic crystal cavity. In: Nature Communications, vol. 6, no. 1, pp. 7665, 2015, ISSN: 2041-1723. (Type: Journal Article | | | )|
The integration of InAs/GaAs quantum dots into nanophotonic cavities has led to impressive demonstrations of cavity quantum electrodynamics. However, these demonstrations are primarily based on two-level excitonic systems. Efforts to couple long-lived quantum dot electron spin states with a cavity are only now succeeding. Here we report a two-spin-cavity system, achieved by embedding an InAs quantum dot molecule within a photonic crystal cavity. With this system we obtain a spin singlet-triplet $Łambda$-system where the ground-state spin splitting exceeds the cavity linewidth by an order of magnitude. This allows us to observe cavity-stimulated Raman emission that is highly spin-selective. Moreover, we demonstrate the first cases of cavity-enhanced optical nonlinearities in a solid-state $Łambda$-system. This provides an all-optical, local method to control the spin exchange splitting. Incorporation of a highly engineerable quantum dot molecule into the photonic crystal architecture advances prospects for a quantum network.
| Turk, Michael E; Vora, Patrick M; Fafarman, Aaron T; Diroll, Benjamin T; Murray, Christopher B; Kagan, Cherie R; Kikkawa, James M: Ultrafast Electron Trapping in Ligand-Exchanged Quantum Dot Assemblies. In: ACS Nano, vol. 9, no. 2, pp. 1440–1447, 2015, ISSN: 1936-0851. (Type: Journal Article | | | )|
S emiconductor quantum dots (QDs) have been the focus of intense study for their size-tunable properties with applications in light-emitting devices, photo-voltaic devices, and printable electronics. 1À6 The size, shape, composition, and surface chemistry of QDs, as well as their tempera-ture and local environment, are all of great importance, modifying photoluminescence (PL) yields, shifting optical transitions, and impacting electrical conductivity. 7À18 Time-resolved optical spectroscopies can directly monitor relaxation pathways that respond sen-sitively to these parameters, and there exists a rich literature measuring the population-averaged response of QDs dispersed in solu-tions and polymer matrices. 19À24 To date, ultrafast optical studies of close-packed col-loidal QD solids have been more limited in number, 15,25À29 and although the majority of these studies have typically been performed at room temperature, measurements at low temperature offer the potential for increased sensitivity to environmental changes as well as a sharper view of relaxation processes. In this report, we study the impact of surface treatment on the low temperature (10 K) optical properties of CdSe QD solids. The ligand exchange and annealing process used here 13 has laid the foundation for high mobility CdSe field effect transitors, where the role of surface traps is underscored by the need to use indium to passivate the QD surfaces and to increase the Fermi energy in order to reduce a mobility gap. 16,30 Here we employ time-resolved absorption (TRA) and photoluminescence (TRPL) spectroscopies to gain insight into the ligand exchange and annealing process through excited state dy-namics. Because both TRA and TRPL imple-mentations developed here are broadband with subpicosecond time resolution, we are able to show that exchanging aliphatic na-tive ligands (NL) for thiocyanate (SCN) and subsequently annealing the samples in-creases electron trapping rates by 2 orders of magnitude. We demonstrate a need to modify the conventional interpretation of CdSe QD time-resolved absorption spectra in this experimental configuration and show that the electron, not the hole, traps first out of the core excitonic state. ABSTRACT We use time-integrated and time-resolved photoluminescence and absorption to characterize the low-temperature optical properties of CdSe
| Sweeney, Timothy M; Carter, Samuel G; Bracker, Allan S; Kim, Mijin; Kim, Chul Soo; Yang, Lily; Vora, Patrick M; Brereton, Peter G; Cleveland, Erin R; Gammon, Daniel: Cavity-stimulated Raman emission from a single quantum dot spin. In: Nature Photonics, vol. 8, no. 6, pp. 442–447, 2014, ISSN: 1749-4885. (Type: Journal Article | | | )|
Solid-state quantum emitters have shown strong potential for applications in quantum information, but the spectral inhomogeneity of these emitters poses a significant challenge. We address this issue in a cavity-quantum dot system by demonstrating cavity-stimulated Raman spin flip emission. This process avoids populating the excited state of the emitter and generates a photon that is Raman shifted from the laser and enhanced by the cavity. The emission is spectrally narrow and tunable over a range of at least 125 GHz, which is two orders of magnitude greater than the natural linewidth. We obtain the regime in which the Raman emission is spin dependent, which couples the photon to a long-lived electron spin qubit. This process can enable an efficient, tunable source of indistinguishable photons and deterministic entanglement of distant spin qubits in a photonic-crystal quantum network.
| Yakes, Michael K; Yang, Lily; Bracker, Allan S; Sweeney, Timothy M; Brereton, Peter G; Kim, Mijin; Kim, Chul Soo; Vora, Patrick M; Park, Doewon; Carter, Samuel G; Gammon, Daniel: Leveraging Crystal Anisotropy for Deterministic Growth of InAs Quantum Dots with Narrow Optical Linewidths. In: Nano Letters, vol. 13, no. 10, pp. 4870–4875, 2013, ISSN: 1530-6984. (Type: Journal Article | | | )|
Crystal growth anisotropy in molecular beam epitaxy usually prevents deterministic nucleation of individual quantum dots when a thick GaAs buffer is grown over a nanopatterned substrate. Here, we demonstrate how this anisotropy can actually be used to mold nucleation sites for single dots on a much thicker buffer than has been achieved by conventional techniques. This approach greatly suppresses the problem of defect-induced line broadening for single quantum dots in a charge-tunable device, giving state-of-the-art optical linewidths for a system widely studied as a spin qubit for quantum information.
| Mesquita, Rickson C; Schenkel, Steven S; Minkoff, David L; Lu, Xiangping; Favilla, Christopher G; Vora, Patrick M; Busch, David R; Chandra, Malavika; Greenberg, Joel H; Detre, John A; Yodh, A G: Influence of probe pressure on the diffuse correlation spectroscopy blood flow signal: extra-cerebral contributions. In: Biomedical Optics Express, vol. 4, no. 7, pp. 978, 2013, ISSN: 2156-7085. (Type: Journal Article | | | )|
A pilot study explores relative contributions of extra-cerebral (scalp/skull) versus brain (cerebral) tissues to the blood flow index determined by diffuse correlation spectroscopy (DCS). Microvascular DCS flow measurements were made on the head during baseline and breath-holding/hyperventilation tasks, both with and without pressure. Baseline (resting) data enabled estimation of extra-cerebral flow signals and their pressure dependencies. A simple two-component model was used to derive baseline and activated cerebral blood flow (CBF) signals, and the DCS flow indices were also cross-correlated with concurrent Transcranial Doppler Ultrasound (TCD) blood velocity measurements. The study suggests new pressure-dependent experimental paradigms for elucidation of blood flow contributions from extra-cerebral and cerebral tissues.
| Vora, P M; Torrens, O N; Kikkawa, J M: Calculation of the chirality-dependent orbital magnetic anisotropy in doped semiconducting single-walled carbon nanotubes. In: Carbon, vol. 50, no. 3, pp. 771–777, 2012, ISSN: 00086223. (Type: Journal Article | | | )|
We calculate the orbital magnetic anisotropy ($Delta$$chi$) of semiconducting single-walled carbon nanotubes at different carrier densities using a nearest-neighbor tight-binding model. Kataura plots of $Delta$$chi$ exhibit 2n + m family groupings and chiral index dependence at all carrier densities which are consistent with the trigonal warping effect. The diameter dependence of $Delta$$chi$ varies strongly with carrier density. We fit our data with a symmetry-restricted model to obtain approximate analytic expressions for $Delta$$chi$ as a function of nanotube chirality and carrier density. Our results illustrate the important role of doping on the magnetic properties of carbon nanotubes. Experimental studies of $Delta$$chi$ should take these effects into account for accurate interpretation of their results. textcopyright 2011 Elsevier Ltd. All rights reserved.
| Vora, P M; Gopu, P; Rosario-Canales, M; Pérez, C R; Gogotsi, Y; Santiago-Avilés, J J; Kikkawa, J M: Correlating magnetotransport and diamagnetism of <math display="inline"> <mrow> <mi>s</mi> <msup> <mi>p</mi> <mn>2</mn> </msup> </mrow> </math> -bonded carbon networks through the metal-insulator transition. In: Physical Review B, vol. 84, no. 15, pp. 155114, 2011, ISSN: 1098-0121. (Type: Journal Article | | | )|
Titanium-carbide-derived carbons (TiC-CDCs) are porous sp(2)-bonded networks synthesized by exposing TiC to chlorine gas at an elevated temperature. The latter "chlorination temperature" adjusts the size of the pores and the sp(2)-bonded carbon domains within this material. We perform magnetoresistance, electronic transport, and superconducting quantum interference device magnetization measurements on TiC-CDC samples prepared at different chlorination temperatures. Transport reveals a metal-insulator transition where high (low) chlorination temperature samples are on the metallic (insulating) side of the transition. Magnetoresistance measurements are consistent with transport in the weak and strong localization regimes for metallic and insulating samples, respectively. Changes in diamagnetism, electronic transport, and magnetoresistance data across the metal-insulator transition are coordinated, suggesting that all three properties are controlled by a single parameter, likely the expansion of sp(2)-bonded domains.
| White, Sadie I; Vora, Patrick M; Kikkawa, James M; Winey, Karen I: Resistive Switching in Bulk Silver Nanowire-Polystyrene Composites. In: Advanced Functional Materials, vol. 21, no. 2, pp. 233–240, 2011, ISSN: 1616301X. (Type: Journal Article | | )|
| White, Sadie I; Vora, Patrick M; Kikkawa, James M; Fischer, John E; Winey, Karen I: Temperature-Dependent Resistive Switching in Bulk Silver Nanowire−Polystyrene Composites. In: The Journal of Physical Chemistry C, vol. 114, no. 50, pp. 22106–22112, 2010, ISSN: 1932-7447. (Type: Journal Article | | | )|
Traditionally, bulk nanocomposites of electrically conducting particles and insulating polymers have been categorized as either insulating or conducting when the nanoparticle concentration is below or above the percolation threshold, respectively. Meanwhile, thin-film polymer nanocomposites can exhibit resistive switching behavior appropriate for digital memory applications. Here, we present the first report of reversible resistive switching in bulk, glassy polymer nanocomposites. At compositions close to the electrical percolation threshold measured at low voltage, silver nanowire-polystyrene nanocomposites demonstrate reversible resistive switching with increasing voltage at room temperature. Nanocomposites with compositions outside of this range exhibit either irreversible switching, or no switching at all. We propose that resistive switching in these materials is the result of the field-induced formation of silver filaments that bridge adjacent nanowire clusters, extending the percolation network and decreasing the sample's bulk resistivity. These findings break from the usual dichotomy of insulating or conducting properties in polymer nanocomposites and could inspire new devices that capitalize on this responsive behavior in these versatile materials.
| White, Sadie I; Mutiso, Rose M; Vora, Patrick M; Jahnke, David; Hsu, Sam; Kikkawa, James M; Li, Ju; Fischer, John E; Winey, Karen I: Electrical Percolation Behavior in Silver Nanowire-Polystyrene Composites: Simulation and Experiment. In: Advanced Functional Materials, vol. 20, no. 16, pp. 2709–2716, 2010, ISSN: 1616301X. (Type: Journal Article | | | )|
The design and preparation of isotropic silver nanowire-polystyrene composites is described, in which the nanowires have finite L/D (< 35) and narrow L/D distribution. These model composites allow the L/D dependence of the electrical percolation threshold, ϕc, to be isolated for finite-L/D particles. Experimental ϕc values decrease with increasing L/D, as predicted qualitatively by analytical percolation models. However, quantitative agreement between experimental data and both soft-core and core–shell analytical models is not achieved, because both models are strictly accurate only in the infinite-L/D limit. To address this analytical limitation, a soft-core simulation method to calculate ϕc and network conductivity for cylinders with finite L/D are developed. Our simulated ϕc results agree strongly with our experimental data, suggesting i) that the infinite-aspect-ratio assumption cannot safely be made for experimental networks of particles with L/D < 35 and ii) in predicting ϕc, the soft-core model makes a less significant assumption than the infinite-L/D models do. The demonstrated capability of the simulations to predict ϕc in the finite-L/D regime will allow researchers to optimize the electrical properties of polymer nanocomposites of finite-L/D particles.
| Dong, Angang; Chen, Jun; Vora, Patrick M; Kikkawa, James M; Murray, Christopher B: Binary nanocrystal superlattice membranes self-assembled at the liquid–air interface. In: Nature, vol. 466, no. 7305, pp. 474–477, 2010, ISSN: 0028-0836. (Type: Journal Article | | | )|
The spontaneous organization of multicomponent micrometre-sized colloids or nanocrystals into superlattices is of scientific importance for understanding the assembly process on the nanometre scale and is of great interest for bottom-up fabrication of functional devices. In particular, co-assembly of two types of nanocrystal into binary nanocrystal superlattices (BNSLs) has recently attracted significant attention, as this provides a low-cost, programmable way to design metamaterials with precisely controlled properties that arise from the organization and interactions of the constituent nanocrystal components. Although challenging, the ability to grow and manipulate large-scale BNSLs is critical for extensive exploration of this new class of material. Here we report a general method of growing centimetre-scale, uniform membranes of BNSLs that can readily be transferred to arbitrary substrates. Our method is based on the liquid-air interfacial assembly of multicomponent nanocrystals and circumvents the limitations associated with the current assembly strategies, allowing integration of BNSLs on any substrate for the fabrication of nanocrystal-based devices. We demonstrate the construction of magnetoresistive devices by incorporating large-area (1.5 mm x 2.5 mm) BNSL membranes; their magnetotransport measurements clearly show that device magnetoresistance is dependent on the structure (stoichiometry) of the BNSLs. The ability to transfer BNSLs also allows the construction of free-standing membranes and other complex architectures that have not been accessible previously.
| Vora, P M; Tu, X; Mele, E J; Zheng, M; Kikkawa, J M: Chirality dependence of the <math display="inline"> <mi>K</mi> </math> -momentum dark excitons in carbon nanotubes. In: Physical Review B, vol. 81, no. 15, pp. 155123, 2010, ISSN: 1098-0121. (Type: Journal Article | | | )|
Using a collection of twelve semiconducting carbon nanotube samples, each highly enriched in a single chirality, we study the chirality dependence of the $K$-momentum dark singlet exciton using phonon sideband optical spectroscopy. Measurements of bright absorptive and emissive sidebands of this finite momentum exciton identify its energy as 20 - 38 meV above the bright singlet exciton, a separation that exhibits systematic dependencies on tube diameter, $2n+m$ family, and semiconducting type. We present calculations that explain how chiral angle dependence in this energy separation relates to the Coulomb exchange interaction, and elaborate the dominance of the $K_A_1'$ phonon sidebands over the zone-center phonon sidebands over a wide range of chiralities. The Kataura plot arising from these data is qualitatively well described by theory, but the energy separation between the sidebands shows a larger chiral dependence than predicted. This latter observation may indicate a larger dispersion for the associated phonon near the $K$ point than expected from finite distance force modeling.
| Luo, Zhengtang; Vora, Patrick M; Mele, Eugene J; Johnson, Charlie A T; Kikkawa, James M: Photoluminescence and band gap modulation in graphene oxide. In: Applied Physics Letters, vol. 94, no. 11, pp. 111909, 2009, ISSN: 0003-6951. (Type: Journal Article | | | )|
We report broadband visible photoluminescence from solid graphene oxide, and modifications of the emission spectrum by progressive chemical reduction. The data suggest a gapping of the two-dimensional electronic system by removal of pi-electrons. We discuss possible gapping mechanisms, and propose that a Kekule pattern of bond distortions may account for the observed behavior.