Depth Profiling of Langmuir−Blodgett Films with a Buckminsterfullerene Probe

Sostarecz, Audra G.; McQuaw, Carolyn M.; Wucher, A.; Winograd, Nicholas

doi:10.1021/ac0492665

Cited by 51 publications

(58 citation statements)

References 39 publications

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“…29,30 As a first approximation, the overall erosion rate ż is linearly interpolated between the values ż Chol and ż Si of the pure cholesterol film and the silicon substrate, respectively. This is done using both (8) with weighing factors and where S is the integrated peak intensity of the indicated species and S max denotes the respective maximum signal detected throughout the entire depth profile. In the case of S Chol , we use the quasi-steady-state signal extrapolated to zero fluence for S max in order to account for the initial signal drop caused by chemical damage.…”

Section: Interface Widthmentioning

confidence: 99%

Energy Deposition during Molecular Depth Profiling Experiments with Cluster Ion Beams

2008

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The role of the location of energy deposition during cluster ion bombardment on the quality of molecular depth profiling was examined by varying the incident angle geometry. Cholesterol films ∼300 nm in thickness deposited onto silicon substrates were eroded using 40-keV C 60 + at incident angles ranging from 5° to 73° with respect to the surface normal. The erosion process was evaluated by determining at each incident angle the total sputtering yield of cholesterol molecules, the damage cross section of the cholesterol molecules, the altered layer thickness within the solid, the sputter yield decay in the quasi-steady-state sputter regime, and the interface width between the cholesterol film and the silicon substrate. The results show that the total sputtering yield is largest relative to the product of the damage cross section and the altered layer thickness at 73° incidence, suggesting that the amount of chemical damage accumulated is least when glancing incident geometries are used. Moreover, the signal decay in the quasi-steady-state sputter regime is observed to be smallest at offnormal and glancing incident geometries. To elucidate the signal decay at near-normal incidence, an extension to an erosion model is introduced in which a fluence-dependent decay in sputter yield is incorporated for the quasi-steady-state regime. Last, interface width calculations indicate that at glancing incidence the damaged depth within the solid is smallest. Collectively, the measurements suggest that decreased chemical damage is not necessarily dependent upon an increased sputter yield or a decreased damage cross section but instead dependent upon depositing the incident energy nearer the solid surface resulting in a smaller altered layer thickness. Hence, glancing incident angles are best suited for maintaining chemical information during molecular depth profiling using 40-keV C 60 + .The addition of cluster ion beam sources to traditional secondary ion mass spectrometry (SIMS) experiments has expanded the options for new applications. 1-5 One of the most important observations associated with these projectiles is that there is often very little chemical damage buildup during the interaction of the cluster with a molecular solid. 1-10 This effect is generally different from the behavior observed using atomic projectiles where damage buildup is usually quite severe, and the experiments must be carried out in either a low dose or very low kinetic energy (>200 eV) mode in order to retain the desired spectral information. [1][2][3][4][5]8,10,11 The high cleanup efficiency of cluster projectiles opens the possibility of molecular depth profiling through molecular solids, with many examples being reported in the past few years. 6-10,12-18 Buckminsterfullerene (C 60 ) has been shown to be particularly effective in this regard, although other cluster projectiles have also been shown to yield acceptable results. 12,14,16 If the cluster beam is focused to a submicrometer spot, it is feasible to create three-dimensional * To...

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Section: Interface Widthmentioning

confidence: 99%

Energy Deposition during Molecular Depth Profiling Experiments with Cluster Ion Beams

2008

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“…Depth profiling of LB films was performed by a TOF-SIMS instrument described previously [5]. The system is equipped with a 40 keV C 60 ϩ ion source (Ionoptika; Southampton, U.K.), which is directed to the target at an angle of 40°relative to the surface normal.…”

Section: Instrumentationmentioning

confidence: 99%

Chemically alternating langmuir-blodgett thin films as a model for molecular depth profiling by mass spectrometry

Zheng

Wucher

Winograd

2008

J. Am. Soc. Mass Spectrom.

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Langmuir-Blodgett multilayers of alternating barium arachidate and barium dimyristoyl phosphatidate are characterized by secondary ion mass spectrometry employing a 40 keV buckminsterfullerene (C 60 ) ion source. These films exhibit well-defined structures with minimal chemical mixing between layers, making them an intriguing platform to study fundamental issues associated with molecular depth profiling. The experiments were performed using three different substrates of 306 nm, 177 nm, and 90 nm in thickness, each containing six subunits with alternating chemistry. The molecular subunits are successfully resolved for the 306 nm and 177 nm films by cluster ion depth profiling at cryogenic temperatures. In the depth profile, very little degradation was found for the molecular ion signal of the underneath layers compared with that of the top layer, indicating that the formation of chemical damage is removed as rapidly as it is formed. The resolving power decreases as the thickness of the alternating subunits decrease, allowing a depth resolution of 20 to 25 nm to be achieved. The results show the potential of LB films as an experimental model system for studying fundamental features of molecular depth profiling. (J Am Soc Mass Spectrom 2008, 19, 96 -102)

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“…For example, these sources can be focused onto the sample with a probe size about 1 micron, allowing greatly improved molecule-specific imaging experiments. The high secondary ion yield associated with the cluster/solid interaction also allows for molecular depth profiling studies without the accompanying damage accumulation normally associated with atomic bombardment [7][8][9][10][11][12][13][14][15][16][17].…”

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confidence: 99%

Energetic ion bombardment of Ag surfaces by C₆₀⁺ and Ga⁺ projectiles

Sun

Szakal

Winograd

et al. 2005

J. Am. Soc. Mass Spectrom.

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The ion bombardment-induced release of particles from a metal surface is investigated using energetic fullerene cluster ions as projectiles. The total sputter yield as well as partial yields of neutral and charged monomers and clusters leaving the surface are measured and compared with corresponding data obtained with atomic projectile ions of similar impact kinetic energy. It is found that all yields are enhanced by about one order of magnitude under bombardment with the C 60 ϩ cluster projectiles compared with Ga ϩ ions. In contrast, the electronic excitation processes determining the secondary ion formation probability are unaffected. The kinetic energy spectra of sputtered particles exhibit characteristic differences which reflect the largely different nature of the sputtering process for both types of projectiles. In particular, it is found that under C 60 ϩ impact (1) the energy spectrum of sputtered atoms peaks at significantly lower kinetic energies than for Ga ϩ bombardment and (2) the velocity spectra of monomers and dimers are virtually identical, a finding which is in pronounced contrast to all published data obtained for atomic projectiles. T he elucidation of the mechanism of interaction of energetic polyatomic or cluster ions with surfaces is of current interest since these projectiles are now being extensively employed as desorption probes in secondary ion mass spectrometry (SIMS) experiments [1,2]. Because of their propensity to produce higher molecular ion signals than corresponding atomic ions and the emergence of commercially available C 60 ϩ [3,4] and Au 3 ϩ [5, 6] ion guns, applications have expanded dramatically. For example, these sources can be focused onto the sample with a probe size about 1 micron, allowing greatly improved molecule-specific imaging experiments. The high secondary ion yield associated with the cluster/solid interaction also allows for molecular depth profiling studies without the accompanying damage accumulation normally associated with atomic bombardment [7][8][9][10][11][12][13][14][15][16][17].The mechanisms associated with the observed secondary ion yield enhancement are not yet clear, although details are emerging quickly. A basic question that must be resolved is to determine whether the yield enhancement occurs as a consequence of increased ionization efficiency, or is due to more effective sputtering in the neutral desorption channel. There is mixed information about this point. For In and Ag surfaces, for example, it has been shown that the enhanced secondary ion yield under SF m ϩ bombardment (m ϭ 1-5) largely arises from the enhanced ionization efficiency attributable to implanted F atoms [18]. For organic systems, however, very high removal rates of neutral species have been reported [2], obviating the need to invoke enhancement of the secondary ionization probability. Most recently, yields and velocity distributions were measured for In and In 2 sputtered from In surfaces bombarded with Au ϩ , Au 2 ϩ , and Au 3 ϩ [19]. Although these experiments do not ad...

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Depth Profiling of Langmuir−Blodgett Films with a Buckminsterfullerene Probe

Cited by 51 publications

References 39 publications

Energy Deposition during Molecular Depth Profiling Experiments with Cluster Ion Beams

Energy Deposition during Molecular Depth Profiling Experiments with Cluster Ion Beams

Chemically alternating langmuir-blodgett thin films as a model for molecular depth profiling by mass spectrometry

Energetic ion bombardment of Ag surfaces by C₆₀⁺ and Ga⁺ projectiles

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Depth Profiling of Langmuir−Blodgett Films with a Buckminsterfullerene Probe

Cited by 51 publications

References 39 publications

Energy Deposition during Molecular Depth Profiling Experiments with Cluster Ion Beams

Energy Deposition during Molecular Depth Profiling Experiments with Cluster Ion Beams

Chemically alternating langmuir-blodgett thin films as a model for molecular depth profiling by mass spectrometry

Energetic ion bombardment of Ag surfaces by C60+ and Ga+ projectiles

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Energetic ion bombardment of Ag surfaces by C₆₀⁺ and Ga⁺ projectiles