Publications by authors named "Bartlomiej Czerwinski"

We present results from experiments and molecular dynamics (MD) simulations obtained with C and Au impacting on free-standing graphene, graphene oxide (GO), and graphene-supported molecular layers. The experiments were run on custom-built ToF reflectron mass spectrometers with C and Au-LMIS sources with acceleration potentials generating 50 keV C and 440-540 keV Au. Bombardment-detection was in the same mode as MD simulation, i.e., a sequence of individual projectile impacts with separate collection/identification of the ejecta from each impact in either the forward (transmission) or backward (reflection) direction. For C impacts on single layer graphene, the secondary ion (SI) yields for C and C emitted in transmission are ∼0.1 (10%). Similar yields were observed for analyte-specific ions from submonolayer deposits of phenylalanine. MD simulations show that graphene acts as a trampoline, i.e., they can be ejected without destruction. Another topic investigated dealt with the chemical composition of free-standing GO. The elemental composition was found to be approximately COH. We have also studied the impact of Au clusters on graphene. Again SI yields were high (e.g., 1.25 C/impact). 90-100 Au atoms evaporate off the exiting projectile which experiences an energy loss of ∼72 keV. The latter is a summation of energy spent on rupturing the graphene, ejecting carbon atoms and clusters and a dipole projectile/hole interaction. The charge distribution of the exiting projectiles is ∼50% neutrals and ∼25% either negatively or positively charged. We infer that free-standing graphene enables detection of attomole to zeptomole deposits of analyte via cluster-SI mass spectrometry.

We present the first data on emission of C60- stimulated by single impacts of 50 keV C602+ on the self-assembled molecular layer of C deposited on free standing 2 layer graphene. The yield, Y, of C60- emitted in the transmission direction is 1.7%. To characterize the ejection and ionization of molecules, we have measured the emission of C60- from the surface of bulk C (Y = 3.7%) and from a single layer of C deposited on bulk pyrolytic graphite (Y = 3.3%). To gain insight into the mechanism(s) of ejection, molecular dynamic simulations were performed. The scenario of the energy deposition and ejection of molecules is different for the case of graphene due to the confined volume of projectile-analyte interaction. In the case of 50 keV C602+ impacts on graphene plus C, the C atoms of the projectile collide with those of the target. The knocked-on atoms take on a part of the kinetic energy of the projectile atoms. Another part of the kinetic energy is deposited into the rim around the impact site. The ejection of molecules from the rim is a result of collective movement of the molecules and graphene membrane, where the membrane movement provides the impulse for ejection. The efficient emission of the intact molecular ions implies an effective ionization probability of intact C. The proposed mechanism of ionization involves the tunneling of electrons from the vibrationally exited area around the hole to the ejecta.

Introduction: Sedentary lifestyle and its consequences are becoming a serious problem not only among the elderly but also relate to children. Reduced muscle mass, disorder of normal spinal curves or problems related to the percentage of body fat are part of physical activity limitation. The aim of this study was to assess the relationship between somatic type of the body and spinal curvatures in school children.

Material And Methods: This study included 94 randomly selected children from 1-3 elementary schools grades, 49 girls and 45 boys. The subjects were divided into 3 groups: a group of children with a BMI indicating underweight (BMI <14.5, n=25), a group of children with normal BMI (BMI = 14.5-17.5, n = 33) and children BMI indicating overweight (BMI > 17.5, n = 36). The Posturometr-S was a device used for measurement and evaluation of the formation of the spine in the sagittal plane. There were three angles of inclination of anterior-posterior curvature of the spine identified: the α angle-upper thoracic spine, the β angle-thoraco-lumbar spine and the γ angle-lumbosacral spine.

Results: Analysing the results of the different angles of curvature of the spine in the sagittal plane we can notice a statistically significant difference between the groups only in the a angle.

Conclusion: The own investigations have showed that the somatic type of the body does not affect the shape of the spine in school children assessed using Posturometr-S. Ther investigations are needed.

We present the first data from individual C60 impacting one to four layer graphene at 25 and 50 keV. Negative secondary ions and electrons emitted in transmission were recorded separately from each impact. The yields for C(n)(-) clusters are above 10% for n ≤ 4, they oscillate with electron affinities and decrease exponentially with n. The result can be explained with the aid of MD simulation as a post-collision process where sufficient vibrational energy is accumulated around the rim of the impact hole for sputtering of carbon clusters. The ionization probability can be estimated by comparing experimental yields of C(n)(-) with those of C(n)(0) from MD simulation, where it increases exponentially with n. The ionization probability can be approximated with ejecta from a thermally excited (3700 K) rim damped by cluster fragmentation and electron detachment. The experimental electron probability distributions are Poisson-like. On average, three electrons of thermal energies are emitted per impact. The thermal excitation model invoked for C(n)(-) emission can also explain the emission of electrons. The interaction of C60 with graphene is fundamentally different from impacts on 3D targets. A key characteristic is the high degree of ionization of the ejecta.

Molecular dynamics computer simulations have been employed to model the bombardment of Ag{111} covered with three layers of C6H6 by 15 keV Ga and C60 projectiles. The study is aimed toward examining the mechanism by which molecules are desorbed from surfaces by energetic cluster ion beams and toward elucidating the differences between cluster bombardment and atom bombardment. The results show that the impact of the cluster on the benzene-covered surface leads to molecular desorption during the formation of a mesoscopic scale impact crater via a catapulting mechanism. Because of the high yield of C6H6 with both Ga and C60, the yield enhancement is observed to be consistent with related experimental observations. Specific energy and angle distributions are shown to be associated with the catapult mechanism.

The mechanism of enhanced desorption initiated by 15-keV C60 cluster ion bombardment of a Ag single crystal surface is examined using molecular dynamics computer simulations. The size of the model microcrystallite of 165,000 atoms and the sophistication of the interaction potential function yields data that should be directly comparable with experiment. The C60 model was chosen since this source is now being used in secondary ion mass spectrometry experiments in many laboratories. The results show that a crater is formed on the Ag surface that is approximately 10 nm in diameter, a result very similar to that found for Au3 bombardment of Au. The yield of Ag atoms is approximately 16 times larger than for corresponding atomic bombardment with 15-keV Ga atoms, and the yield of Ag3 is enhanced by a factor of 35. The essential mechanistic reasons for these differences is that the C60 kinetic energy is deposited closer to the surface, with the deeply penetrating energy propagation occurring via a nondestructive pressure wave. The numbers predicted by the model are testable by experiment, and the approach is extendable to include the study of organic overlayers on metals, a situation of growing importance to the SIMS community.