15 Aug 1999-Physical Review B (American Physical Society)-Vol. 60, Iss: 7, pp 4481-4484
Abstract: Persistent spectral hole burning spectroscopy was used to study the size dependence of the confined acoustic phonons in CuCl nanocrystals embedded in silicate glass, NaCl, and KCl. It is found that the energies of the confined acoustic phonons in the nanocrystals in glass and KCl are almost the same, but twice larger than those in NaCl, which were obtained from the Stokes-side acoustic phonon holes with the same excitation energies. The confined acoustic phonons in the nanocrystals in glass and KCl can be well explained in terms of the lowest-frequency vibrational modes calculated on a sphere model with a free boundary condition. However, the energies of the confined acoustic phonons in the nanocrystals in NaCl are lower than the frequencies of the lowest-frequency vibrational modes predicted by a cube model with a free boundary condition. This observation shows that the energies of the confined acoustic phonons in CuCl nanocrystals depend on the size, the shape, and the boundary condition of the nanocrystals.
Persistent spectral hole burning spectroscopy was used to study the size dependence of the confined acoustic phonons in CuCl nanocrystals embedded in silicate glass, NaCl, and KCl.
It is found that the energies of the confined acoustic phonons in the nanocrystals in glass and KCl are almost the same, but twice larger than those in NaCl, which were obtained from the Stokes-side acoustic phonon holes with the same excitation energies.
The confined acoustic phonons in the nanocrystals in glass and KCl can be well explained in terms of the lowest-frequency vibrational modes calculated on a sphere model with a free boundary condition.
The energies of the confined acoustic phonons in the nanocrystals in NaCl are lower than the frequencies of the lowest-frequency vibrational modes predicted by a cube model with a free boundary condition.
This observation shows that the energies of the confined acoustic phonons in CuCl nanocrystals depend on the size, the shape, and the boundary condition of the nanocrystals.
TL;DR: This article formulates a continuum elastic theory that includes the dynamical loading by elastic surface ligands and demonstrates that this model is capable of accurately reproducing the l = 0 phonon energy across a variety of different QD samples, including cores with different ligand identities and epitaxially grown CdSe/CdS core/shell heterostructures.
Abstract: The measured low frequency vibrational energies of some quantum dots (QDs) deviate from the predictions of traditional elastic continuum models. Recent experiments have revealed that these deviations can be tuned by changing the ligands that passivate the QD surface. This observation has led to speculation that these deviations are due to a mass-loading effect of the surface ligands. In this article, we address this speculation by formulating a continuum elastic theory that includes the dynamical loading by elastic surface ligands. We demonstrate that this model is capable of accurately reproducing the l = 0 phonon energy across a variety of different QD samples, including cores with different ligand identities and epitaxially grown CdSe/CdS core/shell heterostructures. We highlight that our model performs well even in the small QD regime, where traditional elastic continuum models are especially prone to failure. Furthermore, we show that our model combined with Raman measurements can be used to infer the elastic properties of surface bound ligands, such as sound velocities and elastic moduli, that are otherwise challenging to measure.
Abstract: Semiconductor nanocrystals, also known as quantum dots (QDs) have been used in solid state light emission applications ranging from fluorescent downconverters to LEDs and lasers, as well as energy generation devices such as solar photovoltaics and thermoelectrics. In order to realize these myriad applications, the fundamental physics of both electronic and vibrational energy transfer must be understood to engineer better device performance. This thesis begins with a general introduction to the physics and chemistry of QDs as well as an introduction to lattice vibrations, including a proposed model for understanding thermal conductivity in solid state QD-based devices. It continues with a discussion of the methods used to understand the photoluminescence and vibrational characteristics of QDs, including spectrallyresolved time-correlated single photon counting measurements to understand QD photoluminescence lifetime as a function of emission wavelength, and low-frequency Raman spectroscopy to measure acoustic phonons in nanocrystal solids. These two chapters serve as an introduction to the ideas and methods used throughout the thesis. In Chapter 3, Förster theory is used in conduction with spectrallyand temporallyresolved photoluminescence spectroscopy to understand the rates of excitonic energy transfer in CdSe/CdZnS core/shell QDs through a calculation of the effective dipoledipole coupling distance known as the Förster radius. This work demonstrated energy transfer rates between donor and acceptor QDs between 10-100 times faster than the predictions based on standard applications of Förster theory, corresponding to an effective Förster radius of 8-9 nm in close packed QD films. Several possible effects, including an enhanced absorption cross section, ordered dipole orientations, or dipolemultipole coupling, can explain the observed difference between our measurements and the Förster theory predictions, demonstrating that several standard assumptions commonly used for calculating QD resonant energy transfer rates must be carefully considered when the QDs are in a thin-film geometry. Chapters 4-5 involve the use of low-frequency Raman spectroscopy to probe acoustic phonons in QDs. These low-frequency acoustic vibrations affect the
TL;DR: The current assignment of the low-frequency Raman modes is based on Lamb’s theory that mainly focuses on the mode frequencies of an elastic vibration of a free isotropic sphere.
Abstract: Since the first observation of surface acoustic modes from silicon nanocrystals (NCs) embedded in silica byDuval et al., low-frequency Raman scattering from NCs has become an important researcharea inmanyfields including semiconductor technologies, medical therapeutics, and biophysics. Recent research has indicated that low-frequency Raman spectroscopy is a feasible non-destructive technique for investigating virus functionalization, for example, introducing viruses on different materials, attaching viruses to quantum dots and carbon nanotubes, and forming multiple superstructures. These superstructures are expected to have important applications in biological science and medicine. However, the current assignment of the low-frequency Raman modes is based on Lamb’s theory that mainly focuses on the mode frequencies of an elastic vibration of a free isotropic sphere. Many studies have demonstrated that the surface acoustic-mode frequencies observed from free NCs and NCembedded matrix systems are consistent with the theoretical prediction. This is understandable because a large number of frequency values can be selected to match the experimental results. At the same time, other studies have unequivocally
Abstract: Color centers in diamond, and in particular the nitrogen-vacancy center (NV center), have proven promising for the emerging field of quantum technologies. In this thesis, new concepts for the manipulation and better understanding of the NV center dynamics will be addressed, along with the proposal of novel fields of application. It comprises (i) a study of the NV coupling to intrinsic acoustical phonons in nanocrystals, (ii) the proposal for the bottom-up creation of NV arrangements on the nanoscale by the ordered arrangement of nanodiamonds directed by biological scaffold structures, (iii) the interferometry with nanodiamonds in search of signatures of quantum gravity and (iv) the creation of an illustrative framework for the analysis and design of hybrid NV-nuclear spin couplings. The NV embedding in the diamond crystal lattice naturally involves the coupling to vibrational degrees of freedom. On the nanoscale, the combination of discrete mode properties with significant phonon coupling strengths has the potential for the coherent coupling of the NV center to selected distinct modes. We analyze the nanodiamond size dependence of the coupling to low-energy acoustical phonon modes, thereby focusing on its (strong) impact revealed in energy shifts on the orbitally distinct NV ground-excited state transition. In this way, the potential for coherent coupling for crystal sizes . 30 nm is shown. Moreover, for the specific example of an elastic sphere, the breathing mode is identified as promising, pairing close to homogeneous interaction with stable mode properties. Schemes for exploiting this phonon mechanism for the conditional manipulation of NV-centers are constructed. This is achieved by introducing the phonon coupling to the ground state by Raman transitions and a direct conditional coupling amongst NV centers is obtained in a dispersive regime. Moreover, several concepts for improving the speed and robustness of such conditional gates are addressed. Scaling NV-centers in a controllable coherently-interacting way remains a crucial milestone for a wide range of quantum applications. In contrast to the implantation approach in bulk crystals, we propose an inverse bottom-up approach by assembling nanodiamonds on the nanoscale in designed biological (protein) lattices. This allows for controlled distances of around ten nanometers, thus overcoming fundamental resolution limitations of implantation techniques imposed by scattering. A complete framework for the implementation of controlled quantum operations in such assembled nanodiamond networks is presented. In particular, we show that the challenging random distribution of the NV symmetry axis, combined with a magnetic field, allows for both beneficial individual addressing and a uniform Ising-type coupling. A detailed study is carried out to analyze the decoherence properties based on a diamond surface noise model; this then leads to the construction of fully decoupled gate interactions by time-addition or second order driving methods. Applications, such as cluster state computation or the simulation of Heisenberg chains are proposed and their viability in this framework is supported by numerical simulations. Interferometry with massive particles has potential in studying decoherence mechanisms, but also for testing fundamental limitations of quantum mechanics. We propose and analyze the interferometry with nanodiamonds in a Ramsey-Bordé setup in view of identifying mass enhanced quantum gravity (QG) modifications of the energy dispersion. A phase suppression mechanism associated with the thermal motion and gravitation turns out to render QG signatures inaccessible in such systems; as a remedy a revised setup based on gravitational momentum inversion is constructed. A careful analysis of the interference pattern for different particles sizes, temperatures and decoherence influences suggests typical nanodiamonds capable to shed light on the controversial discussions in the field of QG. Moreover, simple widely applicable formulas for calculating the interference phase and path contributions are derived. Last, a framework for the interpretation and customization of coherent hyperfine-couplings between a ‘pulsed decoupled’ NV-center and single nuclear spins is introduced. This is based on the filter formalism widely known from decoherence descriptions, with the filter describing the external control on the NV-center. Based on an analysis in the perturbative limit, thereby identifying the relation to semiclassical frameworks and resonance conditions for the pulse spacing, this concept is extended beyond that limitation by relying on ‘sliced evolutions’ of non-equidistant decoupling pulses. This then allows for an intuitive interpretation and design of gate interactions both in the weak and strong coupling limit, thus applicable for single spin sensing and the intended use of nuclear spins as a qubit. List of Publications Parts of this thesis are based on or have been taken from material first published in the following peer-reviewed journals: [A1] A. Albrecht, A. Retzker, F. Jelezko and M. B. Plenio, Coupling of nitrogen vacancy centres in nanodiamonds by means of phonons, New. J. Phys. 15, 083014 (2013), arXiv: 1304.2192. Copyright (2013) by IOP Publishing and Deutsche Physikalische Gesellschaft (Creative Commons Attribution Unported (CC BY 3.0)). Chapter 4. [A2] A. Albrecht, G. Koplovitz, A. Retzker, F. Jelezko, S. Yochelis, D. Porath, Y. Nevo, O. Shoseyov, Y. Paltiel and M. B. Plenio, Self-assembling hybrid diamond-biological quantum devices, New. J. Phys. 16 093002 (2014), arXiv: 1301.1871. Copyright (2014) by IOP Publishing and Deutsche Physikalische Gesellschaft (Creative Commons Attribution Unported (CC BY 3.0)). Chapter 5 and parts of Chapter 3. [A3] A. Albrecht, A. Retzker and M. B. Plenio, Testing quantum gravity by nanodiamond interferometry with nitrogen-vacancy centers, Phys. Rev. A 90 033834 (2014), arXiv: 1403.6038 (2014). Copyright (2014) by the American Physical Society (APS Journals). Chapter 6. Publications not covered in this thesis: [A4] A. Albrecht, A. Retzker, Ch. Wunderlich and M. B. Plenio, Enhancement of laser cooling by the use of magnetic gradients, New. J. Phys. 13, 033009 (2011), arXiv: 1009.2441.
Cites background from "Size dependence of confined acousti..."
...Such modified phonon properties in nanoparticles have been observed in numerous experiments, as in Raman scattering  and spectral hole burning spectroscopy , modified specific heat properties  or by the characteristic scaling of the exitonic dephasing rate in semiconductor nanocrystals ....