Mass Spectrometry

Biography

Biography: Purushottam Chakraborty

Abstract

If alkali metals such as Li, Rb, K, Na, etc. (referred as A in general) are present in the neighbourhood of the probing element (M) on a sample surface, quasi-molecular ions can be formed by the attachment of these alkali ions [(MA)⁺ formation] in the secondary ion mass spectrometry (SIMS) process. Formation of these MA⁺ molecular ions has a strong correlation to the atomic polarizability of the element M. The emission process for the re-sputtered species M⁰ is decoupled from the MA⁺ ion formation process, in analogy with the ion formation in secondary neutral mass spectrometry (SNMS), resulting in a drastic decrease in the conventional ‘matrix effect’ in SIMS. Although the detection of MA⁺ molecular ions in SIMS has found its applicability in direct materials quantification, it generally suffers from a low useful yield. In such cases, detection of (MA)_n⁺ (n = 2, 3, . . ) molecular ions offers a better sensitivity (even by several orders of magnitude), as the yields of such molecular ion complexes have often been found to be higher than that of MA⁺ ions. The recombination coefficient of MA⁺ or MA₂⁺ molecular species depends on the electro-positivity or electro-negativity of the element M, respectively.  Apart from the surface binding energy of the respective uppermost monolayer, the changes in local surface work-function have often been found to play a significant role in the emission of these molecular ions. Although these MA_n⁺ molecular-ion based SIMS has great relevance in the analysis of materials, a complete understanding on the formation mechanisms of these ion-complexes is still lacking. A procedure, based on MA_n⁺-SIMS approach, has been proposed for the accurate germanium quantification in Molecular Beam Epitaxy (MBE)-grown Si_1−xGe_x alloys. The ‘matrix effect’ has been shown to be completely suppressed for all Ge concentrations irrespective of impact Cs⁺ ion energies. The methodology has successfully been applied for direct quantitative composition analysis of various thin film and multilayer structures. Recent study on various ZnO-based nanostructures has successfully been correlated to their photo-catalysis and photoemission responses. The talk will address the complex formation mechanisms of MA_n⁺ molecular ions and potential applications of the MA⁺ - SIMS approach in chemical analysis of low-dimensional materials.