Aloysius A. Gunawan

Rapid facile synthesis of Cu2ZnSnS4 nanocrystals

Cu2ZnSnS4 (CZTS) nanocrystals were synthesized via thermolysis of single-source cation and sulfur precursors copper, zinc and tin diethyldithiocarbamates. The average nanocrystal size could be tuned between 2 nm and 40 nm, by varying the synthesis temperature between 150 °C and 340 °C. The synthesis is rapid and is completed in less than 10 minutes. Characterization by X-ray diffraction, Raman spectroscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy confirm that the nanocrystals are nominally stoichiometric kesterite CZTS. The ∼2 nm nanocrystals synthesized at 150 °C exhibit quantum confinement, with a band gap of 1.67 eV. Larger nanocrystals have the expected bulk CZTS band gap of 1.5 eV. Several micron thick films deposited by drop casting colloidal dispersions of ∼40 nm CZTS nanocrystals were crack-free, while those cast using 5 nm nanocrystals had micron-scale cracks.

Imaging "invisible" dopant atoms in semiconductor nanocrystals

Nanometer-scale semiconductors that contain a few intentionally added impurity atoms can provide new opportunities for controlling electronic properties. However, since the physics of these materials depends strongly on the exact arrangement of the impurities, or dopants, inside the structure, and many impurities of interest cannot be observed with currently available imaging techniques, new methods are needed to determine their location. We combine electron energy loss spectroscopy with annular dark-field scanning transmission electron microscopy (ADF-STEM) to image individual Mn impurities inside ZnSe nanocrystals. While Mn is invisible to conventional ADF-STEM in this host, our experiments and detailed simulations show consistent detection of Mn. Thus, a general path is demonstrated for atomic-scale imaging and identification of individual dopants in a variety of semiconductor nanostructures.

Cu2ZnSnS4 nanocrystal dispersions in polar liquids

Cu(2)ZnSnS(4) (CZTS) nanocrystals sterically stabilized with oleic acid and oleylamine ligands and dispersed in nonpolar organic liquids have been extracted into, and electrostatically stabilized in, polar liquids by covering their surfaces with S(2-).

Ligands in PbSe Nanocrystals: Characterizations and Plasmonic Interactions

Ligands on semiconductor nanocrystals such as PbSe have a critical role in controlling electrical and optical properties of an individual nanocrystal and their assembly. Standard synthesis of PbSe nanocrystals usually leaves oleic acid ligands on the nanocrystal surfaces as size stabilizers. Considered insulating (long chains of carbons), oleic acid is typically replaced with short ligands such as hydrazines to decrease the inter-nanocrystal distances and improve electronic coupling among the neighboring nanocrystals [1]. As a consequence, enhanced electrical conduction was obtained in PbSe nanocrystal films with short conducting hydrazine ligands [2]. Despite their importance, detailed microscopy and spectroscopy (EELS) analysis of the ligands are absent.

Usual and Unusual Effects in ADF-STEM Imaging of Dopant Atom in Crystals

Interest in imaging and identifying individual dopant atoms inside crystalline specimens has a long history in electron microscopy. While both conventional transmission electron microscopes (TEMs) and scanning transmission electron microscopes (STEMs) are capable of imaging an individual atom [1], detecting an individual dopant atom inside the crystal appears to be easiest using an annular dark field detector in STEM. With recent advances in lens aberration correction ADF-STEM imaging has become applicable even for crystalline sample with < 0.1 nm spacing between atomic columns. Imaging dopant atoms in crystalline specimens is complicated by sample-sensitive changes in the incident electron beam due to channelling [2]. However, despite these challenges, several groups have successfully imaged dopant atoms inside a host: Voyles et al. [3] observed Sb atoms inside Si, Varela et al. [4] studied La-doped CaTiO3, Shibata et al. [5] imaged Y atoms in Al2O3, and Lupini et al. [6] studied Bi dopant atoms in Si. Yet, the number of cases reported in literature is too few to develop a systematic view on parameters and conditions that govern visibility of individual dopant atom in ADF-STEM images. Understanding the roles of the microscope parameters and specimen conditions are instrumental in designing experiments to detect individual dopant atoms and determine their location as precisely as possible to reconstruct the atomic structure of the doped material. We have investigated the limits of ADF-STEM imaging to determine presence and position of individual dopant atoms inside the host crystal by analysing simulated using the multislice method ADFSTEM images. Specimen features, such as thickness and crystallographic orientation of the host material, position of a dopant atom inside a specimen, Z-difference between dopant element and host, and probe parameters that affect the visibility of a dopant atom have been examined. This analysis provides a guide for optimization of the conditions for improving detection of a dopant atom and assessment of the conditions under which a dopant atom is not detectable at all in ADF-STEM images. The relationship between the position of a dopant atom and its visibility in ADF-STEM image is not simple: several different locations of dopant atom can lead to the same contrast between doped and non-doped atomic columns. For instance, an Sn dopant atom located on the entrance surface in 3 nm thick Si [110] has 66% visibility. Sn dopant atom, located 2.7 below the entrance surface, in 5 nm thick Si [110] has also 66% visibility. Moreover, different types of dopant atoms can also have the same calculated visibility. For example, a Ge dopant atom, located 1.2 nm below the entrance surface in a 2 nm thick Si oriented long [110] direction also has 66% visibility. Thus, determination of dopant atom position requires at least knowledge of dopant identity and specimen thickness. However, even with specimen thickness, dopant element, and probe conditions known, two different dopant atom positions can still lead to the same visibility (see Fig. 1). Another result showing the extent to which intensity of doped columns can be counter-intuitive is the presence of negative values of visibility at certain specimen thicknesses although ZDopant>ZHost. For instance, Sn dopant atom at 9 nm depth in 25 nm thick Si [111] has -4 \% visibility (see Fig. 2). Comparison of beam intensity profiles of doped and pure columns shows that the beam intensity of a doped column is less than the intensity of a pure column at certain specimen thicknesses, as shown in Fig. 3. Atoms exposed to fewer incident electrons will contribute less to ADF image intensity. Hence, a doped column can have lower intensity than the pure host column even though ZDopant>ZHost. Similarly, a doped column can have higher intensity than a pure column when ZDopant<ZHost (see Fig. 4) [7].