Laser desorption time-of-flight mass spectrometry of atomic switch memory Ge2Sb2Te5 bulk materials and its thin films.

RATIONALE: Although the structure of atomic switch Ge2Sb2Te5 (GST) thin films is well established, the composition of the clusters formed in the plasma plume during pulsed-laser deposition (PLD) is not known. Laser Desorption Ionization Time-of-Flight Mass Spectrometry (LDI-TOF MS) is an effective method for the generation and study of clusters formed by laser ablation of various solids and thus for determining their structural fragments. METHODS: LDI of bulk or PLD-deposited GST thin layers and of various precursors (Ge, Sb, Te, and Ge-Te or Sb-Te mixtures) using a nitrogen laser (337 nm) was applied while the mass spectra were recorded in positive and negative ion modes using a TOF mass spectrometer equipped with a reflectron while the stoichiometry of the clusters formed was determined via isotopic envelope analysis. RESULTS: The singly negatively or positively charged clusters identified from the LDI of GST were Ge, Ge2, GeTe, Ge2Te, Ten (n = 1-3), GeTe2, Ge2Te2, GeTe3, SbTe2, Sb2Te, GeSbTe2, Sb3Te and the low abundance ternary GeSbTe3, while the LDI of germanium telluride yielded Gem Ten (+) clusters (m = 1-3, n = 1-3). Several minor Ge-H clusters were also observed for pure germanium and for germanium telluride. Sbn clusters (n = 1-3) and the formation of binary TeSb, TeSb2 and TeSb3 clusters were detected when Sb2Te3 was examined. CONCLUSIONS: This is the first report that elucidates the stoichiometry of Gem Sbn Tep clusters formed in plasma when bulk or nano-layers of GST material are ablated. The clusters were found to be fragments of the original structure. The results might facilitate the development of PLD technology for this memory phase-change material.

Laser ablation of ternary As-S-Se glasses and time-of-flight mass spectrometric study.

Ternary chalcogenide As-S-Se glasses, important for optics, computers, material science and technological applications, are often made by pulsed laser deposition (PLD) technology but the plasma composition formed during the process is mostly unknown. Therefore, the formation of clusters in a plasma plume from different glasses was followed by laser desorption ionization (LDI) or laser ablation (LA) time-of-flight mass spectrometry (TOF MS) in positive and negative ion modes. The LA of glasses of different composition leads to the formation of a number of binary As(p)S(q), As(p)Se(r) and ternary As(p)S(q)Se(r) singly charged clusters. Series of clusters with the ratio As:chalcogen = 3:3 (As(3)S(3)(+), As(3)S(2)Se(+), As(3)SSe(2)(+)), 3:4 (As(3)S(4)(+), As(3)S(3)Se(+), As(3)S(2)Se(2)(+), As(3)SSe(3)(+), As(3)Se(4)(+)), 3:1 (As(3)S(+), As(3)Se(+)), and 3:2 (As(3)S(2)(+), As(3)SSe(+), As(3)Se(2)(+)), formed from both bulk and PLD-deposited nano-layer glass, were detected. The stoichiometry of the As(p)S(q)Se(r) clusters was determined via isotopic envelope analysis and computer modeling. The structure of the clusters is discussed.

Laser desorption ionization of red phosphorus clusters and their use for mass calibration in time-of-flight mass spectrometry.

Phosphorus clusters P(n) (n = 1-89) are easily formed from red phosphorus by laser desorption ionization (LDI) and they cover a range of up to approx. m/z 3000 in both positive and negative ion mode. The clusters are singly charged and the spectra are simple because phosphorus is monoisotopic. The mass spectra can be measured with an acceptable resolution and intensity. The use of positively charged P(n) clusters for calibration in mass spectrometry was examined and it was demonstrated that in external calibration a standard deviation of +/-0.04 m/z units can be achieved even when using a common commercial matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) instrument. When used as internal standards the P(n) clusters react with some analytes - C(60) and C(70) fullerenes and cucurbituril[8], for example. It was also found that red phosphorus is a suitable MALDI matrix for peptides and proteins, illustrated by the examples of a Calmix mixture of bradykinin, angiotensin, renin, adrenocorticotropic hormone ACTH fragment 18-359 and insulin, and of insulin alone.

Laser ablation of AgSbS(2) and cluster analysis by time-of-flight mass spectrometry.

Thin films of AgSbS(2) are important for phase-change memory applications. This solid is deposited by various techniques, such as metal organic chemical vapour deposition or laser ablation deposition, and the structure of AgSbS(2)(s), as either amorphous or crystalline, is already well characterized. The pulsed laser ablation deposition (PLD) of solid AgSbS(2) is also used as a manufacturing process. However, the processes in plasma have not been well studied. We have studied the laser ablation of synthesized AgSbS(2)(s) using a nitrogen laser of 337 nm and the clusters formed in the laser plume were identified. The ablation leads to the formation of various single charged ternary Ag(p)Sb(q)S(r) clusters. Negatively charged AgSbS(4) (-), AgSb(2)S(3) (-), AgSb(2)S(4) (-), AgSb(2)S(5) (-) and positively charged ternary AgSbS(+), AgSb(2)S(+), AgSb(2)S(2) (+), AgSb(2)S(3) (+) clusters were identified. The formation of several singly charged Ag(+), Ag(2) (-), Ag(3) (-), Sb(3) (+), Sb(3) (-), S(8) (+) ions and binary Ag(p)S(r) clusters such as AgSb(2) (-), Ag(3)S(-), SbS(r) (-) (r = 1-5), Sb(2)S(-), Sb(2)S(2) (-), Sb(3)S(r) (-) (r = 1-4) and AgS(2) (+), SbS(+), SbS(2) (+), Sb(2)S(+), Sb(2)S(2) (+), Sb(3)S(r) (+) (r = 1-4), AgSb(2) (+) was also observed. The stoichiometry of the clusters was determined via isotopic envelope analysis and computer modeling. The relation of the composition of the clusters to the crystal structure of AgSbS(2) is discussed.

Mass spectrometry of nanodiamonds.

Detonation nanodiamonds (NDs) were studied by time-of-flight mass spectrometry (TOF MS). The formation of singly charged carbon clusters, C(n) (+), with groups of clusters at n = 1-35, n approximately 160-400 and clusters with n approximately 8000 was observed. On applying either high laser energy or ultrasound, the position and intensity of the maxima change and a new group of clusters at n approximately 70-80 is formed. High carbon clusters consist of an even number of carbons while the percentage of odd-numbered clusters is quite low (< or =5-10%). On increasing the laser energy, the maximum of ionization (at n approximately 200 carbons) is shifted towards the lower m/z values. It is suggested that this is mainly due to the disaggregation of the original NDs. However, the partial destruction of NDs is also possible. The carbon clusters (n approximately 2-35) are partially hydrogenated and the average value of the hydrogenation was 10-30%. Trace impurities in NDs like Li, B, Fe, and others were detected at high laser energy. Several matrices for ionizing NDs were examined and NDs themselves can also be used as a matrix for the ionization of various organic compounds. When NDs were used as a matrix for gold nanoparticles, the formation of various gold carbides Au(m)C(n) was detected and their stoichiometry was determined. It was demonstrated that TOF MS can be used advantageously to analyze NDs, characterize their size distribution, aggregation, presence of trace impurities and surface chemistry.

Laser ablation synthesis of phosphorus sulphides, selenides and ternary Pp Sq Ser clusters from various precursors.

Laser ablation (LA) synthesis with simultaneous time-of-flight mass spectrometric (TOF MS) analysis was used to examine the formation and composition of ternary Pp Sq Ser clusters. Clusters formed by LA of various precursors are singular, binary and ternary. Formation of negative or positive singly charged Pp, Sq and Ser clusters, where Pp + (p = 1-249), Pp* (p = 1-191), Sq* (q = 1-15), Sq + (q = 1-12), Ser* (r = 1-8) and Ser+ (r = 1-9), was identified. High numbers of binary Pp Sq, Sq Ser (35) or ternary Pp Sq Ser (138) clusters were formed by LA synthesis from the various mixtures. Most of the ternary Pp Sq Ser clusters were formed either from a mixture of P4S3 with selenium (grey) or from a mixture of SeS2 with red phosphorus. In total, 138 new ternary Pp Sq Ser clusters were identified. The conditions for the formation of such possible prospective nano-materials are given.