We will show that a Si atom in the outermost layer of GaAs has a bi-stable character much alike the well-known DX-center in AlGaAs. In the ground state the Si atom is
negatively charged and in the excited metastable state it is positively charged. These two charge states are related to a modification of the bond configuration of the Si atom in the GaAs surface layer. The voltage dependence of this bi-stable character can be used to bring the Si atom in either of the two states while probing it with an STM tip. The electrical excitation and relaxation processes were studied by analyzing the current and voltage dependence of the observed Random Telegraph Noise. We have successfully used this to create a memory element based on a single impurity atom. Our low T STM setup allows to illuminate the tunneling area and/or to collect tunneling induced photons from the area below the STM tip. We will show our recent results with the optical manipulation of the bond configuration and corresponding charge state of a single bi-stable Si atom as a function of the excitation wavelength (E.P. Smakman et al. PRB 87, 085414 (2013)). This allowed us to unravel different pathways for the excitation and relaxation processes that are involved in this optical manipulation.

Self-assembled InGaAs/GaAs quantum dots (QDs) doped by a single manganese (Mn) atom, a shallow acceptor in InGaAs material, provide the remarkable possibility to optically probe at the few-spin level the exchange interactions between the Mn 3d⁵ electrons, its bound hole, and the QD-confined carriers (electron or hole). In the last 5 years, they have been investigated in great details by using high-resolution micro-photoluminescence of charge-tunable single Mn-doped QDs as a function of an external magnetic field. From these investigations, we could infer a complete description of Mn-doped QDs with a simplified spin Hamiltonian which essentially includes the 2-particle exchange interactions and the spin-orbit interaction typical of such QDs [1,2], and figure out experiment schemes aiming at the control of a single dopant spin. In particular, due to the few meV sp-d exchange interaction and QD-induced anisotropy, it appears that singly Mn-doped QDs define at low temperature a 2-level quantum system associated to the Mn+hole angular momentum |J_z=+1> or |J_z=-1> which is potentially well isolated from its environnement. We recently demonstrated the resonant optical pumping of these |±1> spin states in an experiment making use of a W scheme of transitions which naturally appears when a longitudinal magnetic field is applied [3]. Optical pumping was achieved via the resonant excitation of the central Λ system associated with the neutral exciton state (X⁰), while the outer transitions of the W allowed the non-destructive readout of the spin polarization from the red-shifted photoluminescence of the charged exciton (X^-). An arbitrary spin preparation in the |+1> or |-1> state characterized by a polarization above 50% was thus evidenced, opening the way toward the coherent manipulation of a single Mn dopant. Interestingly, optical excitation also provides a means to control the coupling between two Mn dopants incorporated in the same QD. Such a 2Mn-doped QD could be identified thanks to its specific signature in magneto-optical spectroscopy of the micro-photoluminescence which clearly evidenced two ferromagnetic and two anti-ferromagnetic configurations of the |±1> spins [4]. From a quantitative analysis based on a spin Hamiltonian, it also appears that both dopants which presented essentially no direct coupling, experience an effective ferromagnetic interaction of about 70 µeV which is mediated by the QD-confined hole. Combining this feature with the possibility to optically control a single dopant spin [3,5] should permit to achieve an optically triggered spin-based CNOT gate.
[1] A. Kudelski et al., Phys. Rev. Lett 99, 247209 (2007).
[2] O. Krebs, E. Benjamin, and A.Lemaître, Phys. Rev. B 80, 165315 (2009).
[3] E. Baudin, E. Benjamin, A. Lemaître, and O. Krebs, Phys. Rev. Lett. 107, 197402 (2011).
[4] O. Krebs and A. Lemaître, Phys. Rev. Lett. 111, 187401 (2013).
[5] D. Thuberg, D. E. Reiter, V. M. Axt, and T. Kuhn, Phys. Rev. B 88, 085312 (2013).

Recent advances in nanofabrication enable semiconductor device sizes small enough that the presence of a single dopant significantly affects the properties of the host[1], or forms the active component of the device[2] . Individual dopants can be precisely positioned in a host lattice[3] and their electronic properties spatially mapped with atomic resolution. Recently, electronic control of the lattice position of individual dopants has been demonstrated, including displacement of a single Si dopant in the surface layer of GaAs from a substitutional to an interstitial site by manipulation of the charge state with a scanning tunneling microscope. This reversible transition[4][5] is similar to the naturally occurring formation of a DX- center in AlGaAs. Magnetic dopants (transition-metal atoms) have internal spin degrees of freedom associated with their core d states, and for Fe impurities in GaAs that core electronic state can be manipulated using the tip of a scanning tunneling microscope[6]. Furthermore, a reversible and hysteretic change in lattice position for a single Fe dopant in GaAs is observed when a negative bias voltage is applied.
To evaluate the possibility of a reversible, electronically-induced shift in the position of an Fe dopant from substitutional to interstitial we have performed first-principle calculations to evaluate the formation energy profile of a single substitutional Fe atom embedded in GaAs using density functional theory. The structure has been evaluated using the linearized augmented plane wave (LAPW) method with the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional within the WIEN2k density functional code. Our calculation predicts that a bi-stable state exists for the Fe dopant in agreement with scanning tunneling microscope measurements. The second stable state is characterized by the displacement of the substitutional Fe atom along the <111> into an interstitial region of the GaAs lattice. The displacement is accompanied by a change in atomic configuration about the Fe from a four-fold to six-fold symmetry. These results expand the range of demonstrated local configurational changes induced electronically to include dopants with internal spin states, and thus may be of use for sensitive control of spin-spin interactions between dopants in semiconductor materials.
[1] P. M. Koenraad and M. E. Flatté, Nature Materials 10, 91 (2011).
[2] M. Fuechsle et al, Nature Nanotechnology 7, 242 (2012).
[3] S. R. Schofield, et al, PRL 91, 136104 (2003).
[4] J. K. Garleff, et al, PRB 84, 075459 (2011).
[5] Z. Yi, et al, J. Phys. Chem. C, 115, 23455 (2011).
[6] J. Bocquel, et al, PRB 87, 075421 (2013).

Recently, remarkable progress has been achieved in describing electronic and magnetic properties of individual dopants in semiconductors, both experimentally [1] and theoretically [2, 3], offering exciting prospects for applications in future electronic devices. In view of potential novel applications, which involve communication between individual magnetic dopants, mediated by the electronic carriers of the host, the focus of this research field has been shifting towards fundamental understanding and control of spin dynamics of these atomic-scale magnetic centers. Importantly, the development of time-resolved spectroscopic techniques has opened up the possibility to probe the dynamics of single spin impurities experimentally [4]. These advances pose new challenges for theory, calling for a fully microscopic time-dependent description of spin dynamics of individual impurities in the solid states environment.
We present results of theoretical investigations of real-time spin dynamics of individual and pairs of transition metal (Mn) impurities in GaAs. Our approach combines the microscopic tight-binding description of substitutional dopants in semicondutors [3] with the time-dependent scheme for simulations of spin dynamics [5], based on the numerical integration of equations of motion for the coupled system of the itinerant electronic degrees of freedom of the host and the localized impurity spins. We study the spin dynamics of impurities in finite clusters containing up to hundred atoms, over time scales of a few hundred femtoseconds. In particular, we calculate explicitly the time-evolution of the impurity spins and electrons of the host upon weak external perturbations. From the Fourier spectra of the time-dependent spin trajectories, we identify the energy scales associated with intrinsic interactions of the system, namely the spin-orbit interaction and the exchange interaction between the impurity spins and the spins of the nearest-neighbor atoms of the host. Furthermore, we investigate the effective dynamical coupling between the spins of two spatially separated Mn impurities, mediated by the host carriers. We find signatures of ferromagnetic coupling between the impurities in the time-evolution of their spins. Finally, we propose a scheme for investigating the spin relaxation of Mn dopants in GaAs, by extending the time-dependent approach for spin dynamics in an isolated conservative system to the case of an open system, with dephasing mechanisms included as an effective interaction between the system and an external bath [5].
[1] A. M. Yakunin et al., Phys. Rev. Lett. 92, 216806 (2004); D. Kitchen et al., Nature 442, 436 (2006).
[2] J.-M. Tang et al., Phys. Rev. Lett. 92, 047201 (2004)
[3] T.O. Strandberg et al., Phys. Rev. B 80, 024425 (2009).
[4] A. A. Khajetoorians et al., Science 339, 55 (2013).
[5] A. Pertsova, M. Stamenova and S. Sanvito, Phys. Rev. B 84, 155436 (2011); J. Phys.: Condens. Matter 25, 105501 (2013).

Scanning tunneling Microscope makes it possible to engineer and probe chains of a few magnetic atoms such as Fe, Mn or Co, deposited on a conducting surface. The properties of these fascinating systems depend on the interplay of 3 energy scales and two integer numbers. The energy scales are the single ion anisotropy, interatomic exchange and Kondo coupling and the integer numbers describe the length of the chain and the mulitiplicity 2S+1 of the spin.
In this talk I will discuss the rich variety of different physical phenomena that can arise depending on these 5 variables. This includes phenomena that have been already observed, such as the emergence of classical behavior in sufficiently long S=2 Fe chains on Cu2N, and others that we predict, such as the emergence of fractionalized S=1/2 excitations at the edges of S=1 Heisenberg chains[2] or the enhancement of spin relaxation, analogous to superradiance, in some instances.

The nitrogen-vacancy (NV) centers in diamond have emerged as a promising platform for prospective solid-state quantum spin technologies. Advanced approaches for quantum spin control, based on the pulse dynamical decoupling, protect the quantum state of the NV center's spin from its environment [1]. With such protection, the NV spin can be used for high-precision measurement of magnetic/electric fields and temperature at nanoscale. The necessary control protocols should decouple the NV spin from its environment, while preserving the coupling to the desired degrees of freedom (which are to be measured) [2,3]. Our recent work on designing, assessing, and implementing such protocols for individual and coupled electronic and nuclear spins in diamond will be presented. It will be shown that the controlled NV spin can detect, one by one, individual nuclear spins located 0.5-1 nm away from the NV center [3], and detect the local temperature with precision of 0.1 K [4]. Extensions of this approach to more demanding experimental situations will be discussed.
[1] G. de Lange et al., Science 330, 60 (2010).
[2] T. van der Sar et al., Nature 484, 82 (2012).
[3] T. H. Taminiau et al., Phys. Rev. Lett. 109, 137602 (2012).
[4] D. M. Toyli et al., Pros. Nat. Acad. Sci. 110, 8417 (2013).

Nuclear magnetic resonance (NMR) is the basis of powerful spectroscopic and imaging techniques, but extension to nanoscale samples has been a longstanding challenge due to the insensitivity of conventional detection methods. We are exploring the use of individual, near-surface nitrogen-vacancy (NV) centers in diamond as atomic-size magnetometers to detect proton NMR in organic material located external to the diamond. Using a combination of electron spin echoes and proton spin manipulation, the NV center senses the nanotesla field fluctuations from the protons, enabling both time-domain and spectroscopic NMR measurements on the nanometer scale.
Work performed in collaboration with H. J. Mamin, M. Kim, M. H. Sherwood, C. T. Rettner, K. Ohno, and D. D. Awschalom

Dopant atoms in semiconductor nano devices have recently received attention due to the variability problems in CMOS and the new opportunities for quantum electronics. The latter is based on the fact that dopants act like hydrogen atoms in a semiconductor vacuum with long coherence times and are compatible with VLSI fabrication techniques. Controlled access to single dopants has been achieved in multiple labs and recently control over the electron and nuclear spin has been demonstrated. Optical control of single qubits is very attractive since it allows for high spectral resolution, is non-local, and allows for long distance coupling but was not available in silicon. Here, we present optical addressing and electrical detection of individual erbium dopants with exceptionally narrow line width. The hyperfine coupling is clearly resolved which paves the way to single shot readout of the nuclear spin. This hybrid approach is a first step towards an optical interface to dopants in silicon. Furthermore, spatially resolved tunnelling experiments will be discussed that reveal the spectrum and wavefunction of single dopants and dopant molecules. Dopants studied up to 30 lattice planes below a Si-vacuum interface reveal the peculiar nature of the semiconductor vacuum. Finally, a solid-state molecule is studied with varying bond distance leading to a crossover from a molecular to a correlated atomic state.

Optically transparent materials doped with rear-earth metal ions have been widely investigated for a laser amplifier application. Dopant semiconductors can also contribute to improve the chemical environment of rear-earth metal ions in optical devices. In this work, we designed a fluoroalkylene-bridged sol-gel monomer by a molecular-level hybridization technique. The resulting hybrid glass can be doped dopants without any phase separation and cracking problems. Subsequently, we doped Er+3/CdSe phases into the highly fluorinated glassy host and then carried out fluorescent experiments. Conventional laser amplifiers based on non-fluorinated glass often fails to produce high lasing performance, partially due to a strong absorption from the OH-group at 1540 nm. The low solubility of dopants and the small absorption cross-section of rear-earth metal ions also limit the performance of amplifying since the doping level of rare-earth ions significantly depends upon the lasing efficiency. In a photoluminescence study of the doped fluorinated glass, a significant enhancement in fluorescent intensity at 1540 nm has been observed. The OH-absorption at 1540 nm was also reduced in the highly fluorinated glassy matrix. Furthermore, the presence of CdSe nanoparticles, by virtue of their low phonon energy, also appears to significantly influence the nature of the surrounding environment of Er+3 ions in the fluorinated glassy host, resulting in the increased fluorescent intensity. It is an unusual molecular-level modification, which shows an improve the chemical environment of fluorescence.

The charge transfer from semiconductor nanocrystals to transition metal oxides (e.g. TiO2, SnO2) is important for applications in quantum dot based optoelectronic devices, such as solar cells, photodetectors, and light emitting diodes. An optimized device would require fast nanocrystal-oxide charge transfer rate, minimized back-transfer / back recombination, and high conductivity of both the nanocrystal thin film and oxide film. One of the most widely used method to tune the conductivity is doping. Herein we discuss the effect of doping the PbS nanocrystals and the TiO2 on the charge transfer and back transfer rate, using transient absorption spectroscopy. Complimentary density of states measurements enable us to explain the charge transfer properties within the framework of Marcus theory.

Solitary group V atoms in silicon have recently gained increasing attention not only due to their usability in highly scaled semiconductor devices but also because of their potential application in future computation concepts such as quantum information processing (QIP) and spintronics. For example, a recent proposal by Stoneham, Fisher, and Greenland [1] suggested that gates for QIP could be fabricated using a trimer composed of two types group V atoms, P and Bi. However, the implications of using different elements with vastly varying covalent radii and electronic potentials have yet to be fully understood.
We present a scanning tunnelling microscopy (STM) study of individual Bi and Sb atoms in the cleaved Si(111)2x1 surface [2,3]. Samples are prepared by combining ion implantation and cross sectional STM, enabling us to study solitary atoms of different elements at the atomic scale. High resolution STM topography images and scanning tunneling spectroscopy (STS) measurements in conjunction with density functional theory (DFT) simulations reveal that individual atoms of different elements form not only different reconstructions in the surface but also display opposite charge states. By comparing Bi, Sb, and the previously studied P [4] in identical crystal positions we are able to delineate the influence of the group V atom on its atomic reconstruction in the surface and demonstrate how this relation is crucially influenced by the atom&’s size. Furthermore, we introduce a model that explains how changes in the surface reconstruction determine the polarity as well as the location of the observed charges in the surface. These findings describe novel atomic scale characteristics of solitary group V donors, which have important implications for the choice of element in the design and the fabrication of future atomic scale devices. They also form part of an overall program to investigate a variety of atomic-scale point defects in semiconductors, including arrays of dangling bonds on the Si(001) surface [5] and subsurface atoms like As [6].
*This work was done in collaboration with Philipp Studer, Kitiphat Sinthiptharakoon, Veronika Brazdova, Steven R. Schofield, David R. Bowler, and Neil J. Curson.
[1] A. M. Stoneham, A. J. Fisher, and P. T. Greenland, J. Phys.: Cond. Matter 15, L447 (2003).
[2] P. Studer et al., ACS Nano 6, 10456 (2012).
[3] P. Studer et al., Appl. Phys. Lett. 102, 012107 (2013).
[4] J. K. Garleff et al., Phys. Rev. B 76, 125322 (2007).
[5] S.R. Schofield et al., Nature Commun. 4, 1649 (2013).
[6] K. Sinthiptharakoon et al., arXiv:1307.6890.

Recent success in demonstrating single donor quantum bits (qubits) in silicon [Pla et al., Nature 2012] has spurred efforts towards fabricating two-qubit structures using donors. Single ion implantation is recognized as one of the primary paths towards realizing this goal. In this talk, we will discuss the fabrication of devices with counted ion implants achieved through using single ion detectors integrated with a gated nanowire single electron transistor structure. These devices were fabricated using 50 keV Sb+ ion implantation counted using avalanche photodiodes running in linear mode. This technique has demonstrated sensitivity to <1 ion/pulse detection with a SNR of 0.7, which is equivalent to 36% error rate. These parameters result in a Sb range of 30 nm below the Si/SiO2 interface with a depth straggle of ~10 nm and the implant is self-aligned to the poly-silicon enhancement gate of the nanowire. Low temperature measurements of a series of counted implants samples with 20, 10 and 5 ions/device will be discussed.
Distinguishing features of the approach discussed in this talk include: (1) Focused Sb ion beam implantation providing the ability to select individual holes in a pattern formed by e-beam lithography, (2) Integrated single ion detectors running in either linear mode (discussed in this abstract) or in Geiger mode to improve the sensitivity to single ion strikes (demonstrated in previous work [Bielejec, Nanotechnology 2010]) and (3) Self-aligned poly-Si processing to allow for accurate positioning of the donor pairs relative to a gate between the donor locations which can be used to modulate the tunnel coupling (i.e., a J-gate [Kane, Nature 1998]). The combination of low energy heavy ion implantation (to minimize depth straggle), electron-beam lithography (to define implantation sites), focused ion beam implantation (to localize the ion implantation to specific sites), and self-aligned poly-Si based gates integrated with single ion detectors ( allowing for high resolution alignment between the donor sites and the coupling gate) will allow for ability to fabricate two donor configurations for the demonstration of a viable two donor qubit system.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

The use of highly enriched ²⁸Si as a host material for ³¹P donors enables optical addressing and manipulation of the electron and nuclear spin states with coherence (T₂) times exceeding 1 s and 180 s respectively. Starting with natural abundance silane, we use mass filtered ion beam deposition to grow epitaxial thin films of ²⁸Si with an isotope fraction > 99.9998 % (< 1 ppm residual ²⁹Si), exceeding previously reported levels of enrichment. ²⁸Si provides a non-interacting “semiconductor vacuum” medium for spin qubits such as ³¹P atoms, enabling longer T₂ times and sharpening the line widths of optical transitions. In natural silicon, the nuclear spins of ²⁹Si atoms (4.7 % abundance) causes the ³¹P optical transitions to be unresolvable as well as provide the dominant mechanism for decoherence of the qubit&’s spin state being manipulated. Of particular interest to donor devices is the manifold of hyperfine transitions of the ³¹P neutral donor exciton and ground state. In ²⁸Si, the ³¹P atoms can be optically prepared and manipulated with high fidelity while the nuclear and electron spins are controlled as a quantum memory and operator respectively. Numerous experimental systems can take advantage of ²⁸Si as a medium for ³¹P donor qubits including scanning tunneling microscope based hydrogen lithography devices, single donors coupled to single electron transistors, and quantum wells. The importance of ²⁸Si to ³¹P quantum information systems and the scarcity of such material make clear the critical need for an alternate source of enriched silicon such as the one we demonstrate.

Recent advances in scanning tunneling microscopy (STM) have allowed its use as a lithographic tool for fabrication of atomically precise devices in silicon. Development of this technique is motivated by areas of research that demand ultimate control over the number and position of dopants in a device, which includes quantum information processing and end-of-Moore&’s law test structures. The technique consists of locally depassivating a hydrogen terminated surface with an STM tip, adsorbing phosphine, incorporating the phosphorus and then subsequently burying the dopants in place with Si epitaxy. Demonstrations of remarkable in-plane nanostructures down to single atom devices have been reported.
An ability to fabricate atomically precise devices that can also form gate tunable channels, such as a MOSFET, is highly desirable. In-plane structures become rapidly crowded in lay-out, for example. However, to maintain strict control over the dopant positions after they are incorporated the thermal budget must be very limited. This limit does not allow the fabrication of a good oxide-silicon interface using standard oxidation techniques. In this talk we propose and discuss experimental results towards realizing a low temperature atomic precision process flow that enables surface gate integration with a high quality interface through use of SiGe and strained-silicon on insulator. In previous work, hydrogen lithography and phosphorus incorporation was successfully demonstrated on strained silicon on insulator (sSOI). Strained silicon on insulator has been shown to be relatively insensitive to thermal relaxation and thereby provides a starting material that satisfies the requirements of both enabling high temperature surface preparation steps combined with providing a strained layer that can be capped with relaxed SiGe forming a high quality interface for gate tunable channel formation. In this talk we will present, STM and other characterization results on cleaning, hydrogen lithography, dopant incorporation and SiGe growth on sSOI.
Acknowledgments:
This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. The work was supported by the Sandia National Laboratories Directed Research and Development Program. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U. S. Department of Energy under Contract No. DE-AC04-94AL85000.

Long-lived electronic and nuclear spin states have made nitrogen-vacancy (NV) defects in diamond a leading candidate for solid-state quantum information processing. Moreover, their coherent optical transitions open opportunities for quantum communication or distributed quantum computing. This talk will consider the motivation and requirements for optically-networked quantum devices, and explore challenges and opportunities for realizing them in diamond. In particular, the resonant excitation and emission in these defect centers enables single shot spin detection as well as observation of two-photon quantum interference; these two capabilities have enabled measurement-based entanglement between remote NV centers. Integration into micro-optical cavities may offer a way towards more efficient entanglement distribution while opening new applications for quantum optics with NV centers.

We present an overview of the current status of topological quantum error correction, the current leading approach to achieving a reliable quantum computation due to its highly experimentally compatible requirements. We describe the impact of a variety of experimental realities such as leakage, loss, correlated noise, crosstalk, and geometric constraints, explaining in detail the extent to which each can be tolerated and consequent goals for existing low qubit-count experiments.

Identification of new hybrid solid state systems may pave the way to a robust platform for scalable quantum information processing. Rare earth ions - particularly lanthanides, are vital components in imaging technologies and quantum information processing (QIP) application due to their narrow optical transitions and extremely long coherence times (T2 of several minutes). However, doping of lanthanides into wide band gap semiconductors remains challenging. In parallel, diamond has recently emerged as one of the most promising materials for quantum technologies due to optical readout and the coherent control of a single electron spin of the nitrogen vacancy center.

In this work, we demonstrate a promising approach to merge these two platforms and incorporate several lanthanides (e.g. Europium (Eu) and Praseodymium) ions into single crystal diamond. We use chemical electrostatic assembly to coat Eu containing complexes on diamond and subsequent chemical vapor deposition growth of pristine diamond crystal. Photoluminescence (PL), cathodoluminescence and time resolved measurements are carried out to prove that the Eu atoms are embedded in the diamond lattice. Furthermore, narrowband fluorescence signal recorded from the Eu doped diamond resembles the atomic transitions of the Eu+3. Computational modeling is presented to support the results and unveils that a single Eu atom in a 3+ charge state neighboring a vacancy is a stable configuration within the diamond bandgap. The versatile technique can be applied for a range of materials and offers an unprecedented chemical control over incorporation of impurities into wide bandgap semiconductors. The hybrid rare-earth-diamond system may be a promising platform for QIP

Generation of single photons has been demonstrated in several systems. However, none of them satisfies all the conditions, e.g. room temperature functionality, telecom wavelength operation, high efficiency, as required for practical applications. Here, we report the fabrication of light-emitting diodes (LEDs) based on intrinsic defects in silicon carbide (SiC). To fabricate our devices we used a standard semiconductor manufacturing technology in combination with high-energy electron irradiation. The room temperature electroluminescence of our LEDs reveals two strong emission bands in the visible and near infrared spectral ranges, associated with two different intrinsic defects.[1] Comparing the electroluminescence and photoluminescence properties of 6H-SiC we could assign the emission at 850-1050 nm to silicon-vacancy related (VSi) defects. By using spin-sensitive ODMR technique we were able to provide strong evidences for S = 3/2 ground state of these defects.[2] We also show that they can be generated at a low and potentially even at a single defect level. Hence, our approach can be used to realize electrically driven single photon source for quantum telecommunication and information processing.
[1] F. Fuchs et al., Sci. Rep. 3, 1637 (2013)
[2] H. Kraus et al., Nat. Phys. (accepted) (2013).

ZnS iis a wide, direct band gap II-VI compound semiconductor with band gap energy (3.72 eV for the cubic zinc blende phase and 3.77 eV for the hexagonal wurtzite phase at 300 K) and a large exciton binding energy (40 meV) [1]. ZnS doped with various rare earth ions are capable to produce blue, green and red luminescence. The band gap is the most important aspects of quantum confinement; the decrease in particle size increases the band gap of semiconductor. As the dimensions of nanocrystalline particles approach the exciton bohr radius, a blue shift occurs due to the quantum confinement phenomenon [2]. Therefore, with the principle of dimensional control, one can tune the band gap to realize red to blue shift in band edge emission and the dopant properties.
Cool white light emitting ZnS: Ce3+ nanophosphors have been prepared by an environment friendly low cost chemical ionic mixing method at room temperature. The inorganic metallic precursors and thioacetamide were used as the raw material. Lithium was used as the charge compensator. The Ce3+ ion concentration was varied as 0.001, 0.01, 0.1 mol to realize the optimum emission. The undoped and (Ce, Li) doped ZnS nanophosphors was prepared. The synthesized samples were subjected to various characteristic studies such as X-ray diffraction (XRD), scanning electron microscopy (SEM), UV - Visible spectroscopy (UV-Vis), fluorescence and FTIR spectroscopy to realize the phase purity, surface morphology, absorption levels, spectral analysis and modes of vibration respectively. Results of XRD, SEM and TEM show the particle size ranges from 20 nm to 60 nm with spherical shape morphology revealed by SEM. The Ce3+ ion shows the emission in the range 350 - 600 nm with two maximas at 424 nm and 550 nm due to the electronic transition from 4f05d1 - 4f1 (2F5/2,7/2) of Ce3+ ion as a result of spin - orbit coupling. Report indicates these two maximas are due to defect emission originating at Vozn and VoS even at interstitial levels. Another emission peak at 585 nm due to electronic transition from 4G (4T1) - 6S (6A1) of Mn2+ is absorbed as a result of spin forbidden transition.
Reference:
[1] Biswas S and Kar S 2008 Nanotechnology 19 045710
[2] G. Ghosh, M. K. Naskar, A. Patra, M. Chatterjee, Optical Materials, 28 (2006) 1047- 1053.