Article pubs.acs.org/JPCC
Determination of Internal Structures of Heterogeneous Nanocrystals Using Variable-Energy Photoemission Spectroscopy Sumanta Mukherjee,† Abhijit Hazarika,† Pralay K. Santra,† Ahmed L. Abdelhady,‡ Mohammad Azad Malik,‡ Mihaela Gorgoi,§ P. O’Brien,‡ O. Karis,∥ and D. D. Sarma*,†,∥,⊥,@ †
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India The School of Chemistry and The School of Materials, The University of Manchester, Manchester M13 9PL, U.K. § Helmholtz Zentrum Berlin für Materialien und Energie GmbH, Albert Einstein Strasse 15, 12489 Berlin, Germany ∥ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden ⊥ Council of Scientific and Industrial Research-Network of Institutes for Solar Energy (CSIR-NISE), New Delhi 110001, India ‡
S Supporting Information *
ABSTRACT: This article describes the determination of the internal structure of heterogeneous nanoparticle systems including inverted core−shell (CdS core and CdSe shell) and alloyed (CdSeS) quantum dots using depth-resolved, variableenergy X-ray photoelectron spectroscopy (XPS). A unique feature of this work is the combination of photoelectron spectroscopy performed at lower X-ray energies (400−700 eV), to achieve surface sensitivity, with bulk sensitive measurements at high photon energies (>2000 eV), thereby providing detailed information about the whole nanoparticle structure with a great accuracy. The use of high photon energies furthermore allows us to investigate nanoparticles much larger than those studied thus far. This capability is a consequence of the much-increased mean free path of the photoelectron achieved at high excitation energies. Our results show that the actual structures of the synthesized nanoparticles are considerably different from the nominal, targeted structures, which can be post facto rationalized in terms of the reactivity of different constituents.
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INTRODUCTION Recently, a large variety of heterostructured nanocrystals (NCs) with diverse internal structures1−11 have been synthesized with a range of potential applications in optoelectronics,1,2 light harvesting,3,4 and medicine.5,6 Depending on their particular applications, they can have widely different internal structures like a simple core−shell7−9 structure, a quantum-dot-quantumwell (QDQW) structure,4,10 or complicated coupled-dot structures.11 To meet the increasing demand for heterogeneous NCs with diverse properties suitable for many applications, synthesis and characterization techniques have made rapid progress. It has been shown recently that physical properties, such as exciton recombination lifetime and blinking properties, of these heterogeneous NC systems depend strongly on their internal interfacial structure.12−14 A detailed understanding of the internal structure is therefore essential for explaining their properties and consequently for designing materials with new functionalities.12−14 The techniques most commonly used to characterize such nanostructures include transmission electron microscopy (TEM), UV−visible absorption spectroscopy (UV−vis), inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD), and a threedimensional atom probe. While all these techniques are very valuable for extracting information about NCs, they often have inherent limitations9,10,15 in providing an adequate description of internal structures of such NCs. For example, conventional TEM can easily differentiate the core−shell nature of a © 2014 American Chemical Society
heterogeneous NC when the TEM Z contrast difference between the core and the shell material is sufficiently high8 but fails when the NC consists of elements having similar TEM scattering cross sections.9,10 X-ray photoelectron spectroscopy (XPS) has been used for the past 15 years10,14−26 to provide qualitative as well as quantitative descriptions of the internal structure of a variety of NCs. Such studies10,14,15 have established that the internal structure of complex heterostructured NCs can be very different from the originally targeted structure and that such knowledge of internal structures is essential for understanding their optical properties.10,14 The XPS studies described above utilize photon energies of
