2019 New's Items

Tunnel-FET Switching is Governed by Non-Lorentzian Spectral Line Shape

Vadlamani, S. K.
Agawal, S.
Limmer, D. T.
Louie, S. G.
Fischer , F. R.
Yablonovitch, E.
2019

In tunnel field-effect transistors (tFETs), the preferred mechanism for switching occurs by alignment ( on ) or misalignment ( off ) of two energy levels or band edges. Unfortunately, energy levels are never perfectly sharp. When a quantum dot interacts with a wire, its energy is broadened. Its actual spectral shape controls the current/voltage response of such transistor switches, from on (aligned) to off (misaligned). The most common model of spectral line shape is the Lorentzian, which falls off as reciprocal energy offset squared. Unfortunately, this is too slow a turnoff,...

Probing the Origin of Photoluminescence Brightening in Graphene Nanoribbons

Alavi, S.
Senkovski, B.
Pfeiffer, M.
Haberer, D.
Fischer, F.
Grüneis, A.
Lindfors, K.
2019

We measure the absolute absorbance of a single layer of seven atom wide armchair graphene nanoribbons and study the influence of laser-induced defects on the absorption spectrum of the ribbons. We find that the absorption spectrum shows a broad peak at approximately 2.4 eV that is attributed to excitonic transitions and a smaller peak at 1.77 eV. The low-energy peak is diminished when we induce defects in the material. Simultaneously the photoluminescence is significantly enhanced. We thus attribute the 1.77 eV spectral feature in the absorption spectrum to a quenching state, which...

Resonance Raman Characterization of Tetracene Monomer and Nanocrystals: Excited State Lattice Distortions with Implications for Efficient Singlet Fission

S. R. Ellis
D. R. Dietze
T. Rangel
F. Brown-Altvater
J. B. Neaton
R. A. Mathies
2019

The characterization of specific phonon modes and exciton states that lead to efficient singlet fission (SF) may be instrumental in the design of the next generation of high-efficiency photovoltaic devices. To this end, we analyze the absolute resonance Raman (RR) cross sections for tetracene (Tc) both as a monomer in solution and as a crystalline solid in an aqueous suspension of nanocrystals. For both systems, a time-dependent wavepacket model is developed that is consistent with the absolute RR cross sections, the magnitude of the absorption cross sections, and the vibronic line...

Gas Phase Synthesis of [4]-helicene

Zhao, L.
Kaiser, R. I.
Xu, B.
Ablikim, U.
Fischer, F. R.
2019

A synthetic route to racemic helicenes via a vinylacetylene mediated gas phase chemistry involving elementary reactions with aryl radicals is presented. In contrast to traditional synthetic routes involving solution chemistry and ionic reaction intermediates, the gas phase synthesis involves a targeted ring annulation involving free radical intermediates. Exploiting the simplest helicene as a benchmark, we show that the gas phase reaction of the 4-phenanthrenyl radical ([C14H9]•) with vinylacetylene (C4H4) yields [4]-helicene (C18H12) along with atomic hydrogen via a low-barrier mechanism...

Edge-Functionalized Graphene Nanoribbon Encapsulation to Enhance Stability and Control Kinetics of Hydrogen Storage Materials

Wan, L.
Cho, E.-S.
Marangoni, T.
Shea, P.
Kang, S.Y.
Rogers, C.
Zaia, E.
Cloke, R.
Wood, B.
Fischer, F.
Urban, J.
Prendergast, D.
2019

Hydrogen is a long-term clean energy carrier that enables completely carbon-free energy production. However, practical implementation of hydrogen fuel technologies is restricted because of lack of safe and high-performing storage materials. Here, we report Mg nanocrystals encapsulated by narrow, bottom-up synthesized graphene nanoribbons (GNRs) as environmentally stable and high-capacity hydrogen storage materials. As an encapsulation medium, GNRs offer similar functionalities as reduced graphene oxide to protect the encapsulated Mg nanocrystals from extensive oxidation, while...

Redefining near-unity luminescence in quantum dots with photothermal threshold quantum yield

David Hanifi
Noah Bronstein
Brent Koscher
Zach Nett
Joseph Swabeck
Kaori Takano
Adam Schwartzberg
Lorenzo Maserati
Koen Vandewal
Yoeri van de Burgt
Alberto Salleo
A. Paul Alivisatos
2019

A variety of optical applications rely on the absorption and reemission of light. The quantum yield of this process often plays an essential role. When the quantum yield deviates from unity by significantly less than 1%, applications such as luminescent concentrators and optical refrigerators become possible. To evaluate such high performance, we develop a measurement technique for luminescence efficiency with sufficient accuracy below one part per thousand. Photothermal threshold quantum yield is based on the quantization of light to minimize overall measurement uncertainty. This...

Highly Active and Stable Single-Atom Cu Catalysts Supported by a Metal–Organic Framework

A. M. Abdel-Mageed
B. Rungtaweevoranit
M. Parlinska-Wojtan
X. Pei
O. M. Yaghi
R. J. Behm, J.
2019

Single-atom catalysts are often considered as the ultimate design principle for supported catalysts, due to their unique geometric and electronic properties and their highly efficient use of precious materials. Here, we report a single-atom catalyst, Cu/UiO-66, prepared by a covalent attachment of Cu atoms to the defect sites at the zirconium oxide clusters of the metal–organic framework (MOF) UiO-66. Kinetic measurements show this catalyst to be highly active and stable under realistic reaction conditions for two important test reactions, the oxidation of CO at temperatures up to...

Manipulating the Transition Dipole Moment of CsPbBr3 Perovskite Nanocrystals for Superior Optical Properties

Matthew Jurow
Thomas Morgenstern
A. Paul Alivisatos
2019

Colloidal cesium lead halide perovskite nanocrystals exhibit unique photophysical properties including high quantum yields, tunable emission colors, and narrow photoluminescence spectra that have marked them as promising light emitters for applications in diverse photonic devices. Randomly oriented transition dipole moments have limited the light outcoupling efficiency of all isotropic light sources, including perovskites. In this report we design and synthesize deep blue emitting, quantum confined, perovskite nanoplates and analyze their optical properties by combining angular...

Ferroelectricity in [111]-oriented epitaxially strained SrTiO3 from first principles

S. E. Reyes-Lillo
K. M. Rabe
J. B. Neaton
2019

We use first-principles density-functional theory calculations to investigate the effect of biaxial strain in the low-temperature structural and ferroelectric properties of [111]-oriented SrTiO3. We find that [111] biaxial strain, achievable by coherent epitaxial growth along the [111] direction, induces structural distortions in SrTiO3...

Bright Sub-20-nm Cathodoluminescent Nanoprobes for Electron Microscopy

Maxim B. Prigozhin
Peter C. Maurer
Alexandra M. Courtis
Nian Liu
A. Paul Alivisatos
et. al.
Nat. Nanotechnol
2019

Electron microscopy has been instrumental in our understanding of complex biological systems. Although electron microscopy reveals cellular morphology with nanoscale resolution, it does not provide information on the location of different types of proteins. An electron-microscopy-based bioimaging technology capable of localizing individual proteins and resolving protein–protein interactions with respect to cellular ultrastructure would provide important insights into the molecular biology of a cell. Here, we synthesize small lanthanide-doped nanoparticles and measure the absolute...