TY  - JOUR
AB  - In semiconductor nanocrystals, excited electrons relax through multiple radiative and nonradiative pathways. This complexity complicates characterization of their decay processes with standard time- and temperature-dependent photoluminescence studies. Here, we exploit a simple nanophotonic approach to augment such measurements and to address open questions related to nanocrystal emission. We place nanocrystals at different distances from a gold reflector to affect radiative rates through variations in the local density of optical states. We apply this approach to spherical CdSe-based nanocrystals to probe the radiative efficiency and polarization properties of the lowest dark and bright excitons by analyzing temperature-dependent emission dynamics. For CdSe-based nanoplatelets, we identify the charge-carrier trapping mechanism responsible for strongly delayed emission. Our method, when combined with careful modeling of the influence of the nanophotonic environment on the relaxation dynamics, offers a versatile strategy to disentangle the complex excited-state decay pathways present in fluorescent nanocrystals as well as other emitters.
AU  - Cocina, A.*
AU  - Brechbühler, R.*
AU  - Vonk, S.J.W.*
AU  - Cui, J.
AU  - Rossinelli, A.A.*
AU  - Rojo, H.*
AU  - Rabouw, F.T.*
AU  - Norris, D.J.*
C1  - 65025
C2  - 52620
SP  - 4145-4151
TI  - Nanophotonic approach to study excited-state dynamics in semiconductor nanocrystals.
JO  - J. Phys. Chem. Lett.
VL  - 13
IS  - 18
PY  - 2022
ER  - 

TY  - JOUR
AB  - Spin-state-selective excitation (S3E) experiments allow the selection of individual transitions in a coupled two spin system. We show that in the solid state, the dipole-dipole interaction (DD) between 15N and 1H in a 1H-15N bond and the chemical shift anisotropy (CSA) of 15N in an amide moiety mutually cancel each other for a particular multiplet component at high field, when the sample is spun off the magic angle (Arctan [√2] = 54.74°). The accuracy of the adjustment of the spinning angle is crucial in conventional experiments. We demonstrate that for S3E experiments, the requirement to spin the sample exactly at the magic angle is not mandatory. Applications of solid state NMR in narrow bore magnets will be facilitated where the adjustment of the magic angle is often difficult. The method opens new perspectives for the development of schemes to determine distances and to quantify dynamics in the solid state.
AU  - Sarkar, R.
AU  - Rodriguez Camargo, D.C.
AU  - Pintacuda, G.*
AU  - Reif, B.
C1  - 47643
C2  - 39420
SP  - 5040-5044
TI  - Restoring resolution in biological solid-state NMR under conditions of off-magic-angle spinning.
JO  - J. Phys. Chem. Lett.
VL  - 6
IS  - 24
PY  - 2015
ER  - 

TY  - JOUR
AB  - We present a novel concept for if pulses and optimal control designed cross-polarization experiments for quadrupolar nuclei. The methods are demonstrated for (2)H CP-MAS and (2)H multiple-pulse NMR of perdeuterated proteins, for which sensitivity enhancements up to an order of magnitude are presented relative to commonly used approaches. The so-called RESPIRATION rf pulses combines the concept of short broad-band pulses with generation of pulses with large flip angles through distribution of the rf pulse over several rotor echoes. This lead to close-to-ideal rf pulses, facilitating implementation of experiments relying on the ability to realize high-performance 90 and 180 degrees pulses, as, for example, in refocused INEPT and double-to-single quantum coherence experiments, or just pulses that provide a true representation of the quadrupolar powder pattern to extract information about the structure or dynamics. The optimal control (2)H ->(13)C CP-MAS method demonstrates transfer efficiencies up to around 85% while being extremely robust toward rf inhomogeneity and resonance offsets.
AU  - Wei, D.*
AU  - Akbey, Ü.*
AU  - Paaske, B.*
AU  - Oschkinat, H.*
AU  - Reif, B.
AU  - Bjerring, M.*
AU  - Nielsen, N.C.*
C1  - 6118
C2  - 28895
CY  - Washington, USA
SP  - 1289-1294
TI  - Optimal 2H rf pulses and 2H-13C cross-polarization methods for solid-state 2H MAS NMR of perdeuterated proteins.
JO  - J. Phys. Chem. Lett.
VL  - 2
IS  - 11
PB  - Amer. Chemical Society
PY  - 2011
ER  -