Positron studies of surfaces, structure and electronic properties of nanocrystals

Abstract

A brief review is given of recent positron studies of metal and semiconductor nanocrystals. The prospects offered by positron annihilation as a sensitive method to access nanocrystal (NC) properties are described and compared with other experimental methods. The tunability of the electronic structure of nanocrystals underlies their great potential for application in many areas. Owing to their large surface-to-volume ratio, the surfaces and interfaces of NCs play a crucial role in determining their properties. Here we focus on positron 2D angular correlation of annihilation radiation (2D-ACAR) and (two-detector) Doppler studies for investigating surfaces and electronic properties of CdSe NCs.1 Introduction Nanoscale particles have become extremely important in the tailoring of materials for a wide range of applications [1][2][3]. In particular, semiconductor nanocrystals (NCs) have attracted wide interest because their electronic and optical properties can be tuned by variation of their size and shape [2,3]. At small sizes, the surface or interface energy becomes a very significant factor in establishing not only the interior crystal structure but also the specific surface structure and ad-atom termination of the NCs. These special properties underlie the large potential of NCs for applications in areas of nanostructured thin-film solar cells [4], opto-electronics [3] and spintronics [5], fission/fusion reactor vessel steels [6], metal hydrides for hydrogen storage [7], and fluorescence-based detection of biomolecules [3].During the last decade, it has been shown that positrons (e + ) act as a sensitive, self-seeking probe of embedded and colloidal NCs [8][9][10][11][12][13][14][15][16][17][18][19][20], revealing the local structural properties and electronic structure via a measurement of the electron momentum density [21]. Positron methods are thus capable of extracting valuable information on properties at specific surface, interface or defect sites in the NCs, which are difficult to access by other methods such as transmission electron microscopy, X-ray diffraction (XRD), and EXAFS. Furthermore, insight into the electronic structure of buried or embedded NC systems is gained, which is difficult to obtain with e.g. STM or XPS.

Divacancies and the hydrogenation of Mg-Ti films with short range chemical order

“…This selective detection requires the presence of chemically segregated Mg areas, into which the positron will trap during its diffusional motion which follows on the implantation and subsequent thermalization process. 16 The trapping is driven by the large difference in positron affinity, by 2.1 eV, 17 for Mg metal relative to Ti ͑Fig. 2͒.…”

“…The absence of a clear signature of Ti in the positron measurements further indicates that the coherent embedding of Ti-domains in Mg leads to interfaces with hardly any vacancy-related defects. Such interface defects would act as positron trapping sites, which are easily detected by the positron method 16,18 via annihilation with 3d-electrons of the surrounding Ti atoms since this would lead to a distinctly higher value of the W parameter than observed for Mg ͑Fig. 1͒.…”

Electronic coupling of colloidal CdSe nanocrystals monitored by thin-film positron-electron momentum density methods

“…In the present study, we apply high-resolution depth-sensitive positron methods 15,[17][18][19] to show that the electronic interaction between CdSe NCs, deposited as thin layers, can be monitored through observation of the electron momentum distribution of the valence states. Further, the depth-resolved positron studies provide insights into further sintering of the nanocrystal layers at higher temperatures.…”

“…1,8,10,11 A recent in situ electron microscopy study on monolayers of PbSe NCs indicates that this is accompanied by rotations of the NCs and the formation of an inorganic interface between neighboring NCs at slightly higher temperatures. 13 Recent studies on semiconductor NCs [14][15][16][17] show the potential of positron methods to study the electronic structure of nanocrystal solids since the positron can be used as a sensitive probe for detecting the surface composition of the nanocrystals and the confinement of the upper valence electron states. In the present study, we apply high-resolution depth-sensitive positron methods 15,[17][18][19] to show that the electronic interaction between CdSe NCs, deposited as thin layers, can be monitored through observation of the electron momentum distribution of the valence states.…”

“…13 Recent studies on semiconductor NCs [14][15][16][17] show the potential of positron methods to study the electronic structure of nanocrystal solids since the positron can be used as a sensitive probe for detecting the surface composition of the nanocrystals and the confinement of the upper valence electron states. In the present study, we apply high-resolution depth-sensitive positron methods 15,[17][18][19] to show that the electronic interaction between CdSe NCs, deposited as thin layers, can be monitored through observation of the electron momentum distribution of the valence states. Further, the depth-resolved positron studies provide insights into further sintering of the nanocrystal layers at higher temperatures.…”