X-ray Photoelectron Spectroscopy X-ray photoelectron spectroscopy (XPS) is a non-destructive technique used to analyze the elemental compositions, chemical and electronic states of materials. XPS has a depth of analysis from 1 to 10nm. Chemical studies can be performed with more depth if surfaces are removed or etched. Samples need to be cleaned and free of surface contamination and fingerprints before conducting an XPS analysis. XPS is performed under ultra-high vacuum conditions and it consists on exposing a material to x-rays and measuring the kinetic energy and number of ejected photo emitted core electrons. The kinetic energy is determined though the next equation: Where: h-planck constant(6.62 x 10-34 J s) v-frequency of the radiation BE: binding energy of ejected electron ᶲ: work function KE = hv -BE + ᶲ Fig 1. XPS system
Fig. 2. XPS Process Characteristics XPS primary beam X-ray analyzed beam electrons (energy) Type of sample all, charging possible area of analysis 10 surface selectivity 1 to 5 nm elemental identification all except H and He sensitivity 0.10% nature of chemical shift, straightforward bombarding depth profiling elemental, chemical destructive nature none if not sputtered Table 1. XPS characteristics Application Examination and identification of surface contaminations Evaluation of materials processing steps (cleaning, plasma etching, thermal oxidation, silicide thin-film formation etc.) Evaluation of thin-film coating or lubricants Failure analysis for adhesion between components (air oxidation, corrosion etc.) Surface composition differences of alloys Example
X-ray Photoelectron Spectroscopy of CdSe Nanocrystals with Applications to Studies of the Nanocrystal Sllrface In this paper the authors present a study of the surface composition of CdSe nanocrystals, with particular attention to the organic groups which cap the nanocrystals and to surface compounds formed after the nanocrystals are synthesized. The authors use XPS to explore the surface properties of semiconductor nanocrystals. They have synthesized nearly monodisperse, crystalline CdSe nanocrystals, using a variation of a technique developed by Murray et al. They bounded nanocrystals to Au and Si surfaces and they performed XPS on a variety of CdSe nanocrystal sizes and to determine the types of surface species present as well as the percent coverage of these species. Figure 1. Attachment of CdSe nanocrystals dissolved in toluene to Au surfaces with hexanedithiol. Most of the samples used in this study were bound to gold surfaces using techniques described elsewhere. Briefly, hexanedithiol is allowed to self-assemble on ion-etched gold evaporated onto glass slides. The slides are then transferred to a solution of nanocrystals, which covalently bond to the sulfur of the hexanedithiol (Figure 1).
Figure 2. Typical XPS survey spectra of CdSe nanocrystals bound to gold using Mg X-ray source. Multiple XPS measurements have been taken on 22 different preparations of CdSe nanocrystals ranging in radius from 9.3 to 30.2 A. Typical survey spectra (Figure 2) were obtained in 5 min using 178-eV pass energy. They show the presence of Au from the substrate, Cd, Se, and P from the nanocrystals and their surfaces, and C and 0 from the nanocrystals surfaces and from absorbed gaseous molecules. The survey spectra give a rough idea of the coverage of nanocrystals on the surface-when the Cd 3d signal is larger than that from Au 4f, the coverage is comparable to Si substrate samples which had coverage of -1/6 monolayer.
Figure 3. (a) Close-up XPS survey spectrum of CdSe clusters deposited from pyridineon gold. (b) Close-up XPS survey spectrum of CdSeclusten deposited from toluene on gold. Note the absence of P for the sample deposited from pyridine.
Figure 4. Close-up XPS spectrum of P 1s core region for a 14-A CdSe nanocrystal deposited on gold from pyridine. The P signal is much lower thanthatshownin Figure8. TheamountofPcorrespondsto 12%coverage of P on the surface. Figure 5. Close-up XPS spectrum of N 1s core region for CdSe nanocrystals deposited on gold from pyridine. Note the absence of signal from N, indicating the absence of pyridine. Figure 6. Oxidation of the CdSe surface CD 3d cores: (a) 15.9-A cluster as prepared; (b) 15.9-A cluster after several weeks in air. Survey spectra of clusters deposited from pyridine indicate an absence of P in the spectrum (Figure 3). The close-up scan of the P region (Figure 4) reveals that a small
amount of P is present. The peak shown fits to an area corresponding to 12% coverage of the nanocrystal surface by P. Thus, while the majority of P has been removed upon dissolution in pyridine, a small amount remains on the nanocrystal surface. Close-up spectra of the N 1s core region (Figure 5), which overlaps with the Cd 3d region, indicate the absence of N on the nanocrystals. The oxidation of Se in the pyridine samples is similar to the oxidation in the toluene samples. However, the pyridine samples alsodevelop an oxidized Cd peak (Figure 6). The peak appeared after the sample had been in air for several weeks. The peak's position agrees with that of oxidized Cd in bulk CdSe. The authors conclude that with the measured XPS spectra on a range of sizes of CdSe nanocrystals covalently bound to gold surfaces. They have seen that the core levels obtained in this way are in agreement with bulk values for CdSe. In addition they have used XPS to probe the surface of CdSe nanocrystals and have determined that the Cd surface sites are passivated by TOP0 and that Se surface sites are predominantly unbonded, in agreement with a recent NMR study. When stored in air, Se surface sites are oxidized, forming an SeOz surface film which is physisorbed to the nanocrystal surface and which degrades over time. In nanocrystals deposited from pyridine, neither the Cd nor Se surface sites are completely passivated. When these are stored in air, both Cd and Se surface sites oxidize. XPS techniques can be readily extended to a variety of soluble semiconductor systems and provide a versatile way to measure the surface composition of semiconductor nanocrystals. References 1. J. E. Bowen Katari, V. L. (1994). X-ray Photoelectron Spectroscopy of CdSe Nanocrystals with Applications to Studies of the Nanocrystal Sllrface. J. Phys. Chem, 4109-41 17. 2. http://webh01.ua.ac.be/mitac4/micro_xpsaes.pdf 3. Horton, D. J. (2009). Dr. J. Hugh Horton. Retrieved from Queens University: Chemistry Department: http://www.chem.queensu.ca/people/faculty/horton/research.html