S.M.M. Ramos

Surface modifications of LiNbO3 single crystals induced by swift heavy ions

Single crystals of LiNbO3 (Y-cut orientation) have been irradiated at GANIL using different ions (112Sn, 155Gd and 238U) accelerated in the GeV range. All the irradiations were performed at room temperature, with fluences extending from 1010 to 1012 ions cm−2. Rutherford backscattering spectrometry in channeling geometry (RBS-C) was employed to investigate the lattice disorder resulting from the high electronic stopping power (dE/dx)e (between 18 and 40 keV nm−1). Surface swelling of the irradiated samples was evidenced using a profilometer, in conjunction with direct observations in the nanometer scale by means of atomic force microscopy (AFM). According to RBS-C analysis, the damage cross section Ad varies from 4 × 10−13 to 1.4 × 10−12cm2 and depends on both (dE/dx)e and the velocity of the incident ions. A correlation was pointed out between the height of the out of plane step and the (dE/dx)e-induced damage. AFM observations, performed on samples irradiated at the lowest fluences, indicate a significant broadening of the latent track radius at the surface.

Latent tracks formation in silicon single crystals irradiated with fullerenes in the electronic regime

Abstract Silicon targets of (1xa00xa00) orientation were prepared for transmission electron microscopy observations and then irradiated with either 30 MeV C602+ or 40 MeV C603+ at normal incidence. All the irradiations were performed at room temperature, up to fluences of a few 109 clusters cm−2. The incident electronic stopping powers were 48 and 57 keV nm−1 for C602+ and C603+ projectiles, respectively. High resolution observations at normal incidence evidenced amorphous zones of circular shape at each projectile impact. The track diameters near the target surface were 8.4 and 10.5 nm for irradiations at 30 and 40 MeV, respectively. This effect, which was never observed in silicon single crystals bombarded with swift heavy ions, was ascribed to the high density of electronic energy associated with the correlated electronic stopping of the cluster components. Observations at conventional resolution of samples tilted in the microscope allowed to follow the depth evolution of the entire tracks. The damage extends from the surface to a maximum depth L which depends on the incident energy of the clusters (L=160 nm and L=190 nm for irradiations at 30 and 40 MeV, respectively). This progressive extinction of the radiation-induced disorder was linked to the decorrelation process of the C60 ions during their slowing-down in the target.

Damage creation in α-Al2O3 by MeV fullerene impacts

Single crystals of α-Al2O3 were irradiated at room temperature with C60 clusters at normal and grazing incidences. The extent of the induced damage was determined using Rutherford backscattering spectrometry in channeling geometry (RBS-C). A damage cross-section of 3.1xa0×xa010−12 cm2 was obtained for the highest electronic stopping power (76.2 keV nm−1). From electron microscopy observations continuous amorphous tracks were evidenced around the projectile trajectory and an electronic stopping power threshold for damage creation of 18 keV nm−1 was also determined. The spatial correlation in depth of the cluster components were deduced from both direct track length measurements and the damage profiles extracted from RBS-C analysis. The maximal correlation length represents about one-third of the projected range (Rp) of a free carbon. Using atomic force microscopy (AFM), conically shaped hillocks corresponding to the out of plane expansion of the latent tracks were observed. These structures characterize nanometric changes of the plastic properties of sapphire induced by high electronic excitations.

Damage induced in LiNbO3 single crystals by GeV gadolinium ions

Abstract Single crystals of LiNbO3 with two different orientations (Y-cut and Z-cut) have been irradiated at GANIL with 5.17 MeV/amu 155Gd ions at room temperature. The fluence extended from 1.5 × 1011 to 1.2 × 1012 ions cm−2. The damage resulting from the high electronic stopping power (up to 30 keV/nm) has been characterized by both Rutherford backscattering ion channeling (RBS-C) and optical absorption measurements. By using a degrader, the defect efficiency has also been investigated as a function of the electronic stopping power (18 keV/nm

Latent track formation in germanium irradiated with 20, 30 and 40 MeV fullerenes in the electronic regime

Abstract Conventional transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) have been performed on irradiated germanium with a few tens of MeV C 60 incident clusters (fullerenes). Normal and inclined incidences of the beam have been investigated. As observed in the case of silicon, microscopy observations after irradiation with 20, 30 and 40 MeV beams show clearly the presence of cylindrical amorphous latent tracks of 6, 10.6 and 12.5 nm in diameter, respectively. Other microstructural considerations are reported. The difference in diameter has been interpreted in terms of high electronic energy deposited by the three different energetic fullerene beams. Furthermore, during HREM observation, a recrystallisation process of the amorphous region of the tracks has been observed and analysed.

Contact angle hysteresis on nano-structured surfaces

We present results from an experimental study on the phenomenon of contact angle hysteresis on solid surfaces decorated by a random array of nanometric hollows. For weak values of the areal density of defects φd, the hysteresis H increases linearly with φd. This evolution is described by a pinning–depinning process of the contact line by individual defects. At higher values of φd, a collective pinning effect appears and H decreases with increasing φd. In the linear regime, our experimental results are compared to theoretical predictions for contact angle hysteresis induced by a single isolated defect on the solid surface. We suggest that the crossover from the individual to the collective pinning effects could be interpreted in terms of an overlapping of wetting cross sections. Finally, we analyse the influence of both the size and the morphology (hollows/hillocks) of defects on the anchorage of the contact line.

Superoleophobic Behavior Induced by Nanofeatures on Oleophilic Surfaces

The control of surface wetting properties to produce robust and strong hydrophobic and oleophobic effects on intrinsically oleophilic surfaces is at the heart of many technological applications. In this paper, we explore the conditions to observe such effects when the roughness of the substrate is of fractal nature and consists of nanofeatures obtained by the ion track etching technique. The wetting properties were investigated using eight different liquids with surface tensions gamma varying from 18 to 72 mN m(-1). While it is observed that all the tested oils readily wet the flat substrates, it is found that the contact angles are systematically exalted on the rough surfaces even for the liquids with very low surface tension. For liquids with gamma > or = 25 mN m(-1) an oleophobic behavior is clearly induced by the nanostructuration. For liquids with gamma < 25 mN m(-1), although the contact angle is enhanced on the nanorough surfaces, it conserves its oleophilic character (theta* lower than 90 degrees). Moreover, our experiments show that even in the case of hexane, liquid having the lowest surface tension, the homogeneous wetting (Wenzel state) is never reached. This high resistance to liquid impregnation is discussed within the framework of recent approaches explaining the wetting properties of superoleophobic surfaces.

High energy heavy ion irradiation effects in α-Al2O3☆

Abstract Single crystals of Al 2 O 3 have been irradiated at GANIL with 3.5 MeV/amu Pb ions, at a temperature of ≌ 8 K. The fluence range extended from 4×10 11 to 1.2×10 12 ions cm −2 . The effects of high electronic excitation induced in the samples have been characterized by Rutherford backscattering on channeling (RBS) in conjunction with optical absorption measurements. Moreover, some samples preliminary implanted with 10 16 57 Fe + ions cm −2 at 110 keV and annealed at 1673 K during one hour were studied using conversion electron Mossbauer specroscopy (CEMS) in order to obtain complementary informations. Preliminary RBS results (77 K irradiations) indicate a damage cross section of ∼ 10 −13 cm 2 , consistent with a track radius of about 1.8 nm. The defect efficiency has been also investigated as a function of the electronic stropping power (d E /D x ) c .

Latent track formation in GaAs irradiated with 20, 30, and 40 MeV fullerenes

Microstructural observations of gallium arsenide single crystals irradiated with a few tens of MeV C60 incident clusters (fullerenes) were performed. Normal and grazing incidences were investigated. Similar to in the case of silicon and germanium, cylindrical amorphous tracks whose diameters vary as a function of the projectile energy were found. However, for a given energy of the clusters, the track diameters are slightly different from one material to another. Also depending on the fullerene, energy is the length of the amorphous cylinder that formed along the projectile’s path. The recrystallization process under an electron beam during transmission electron microscopy observation was analyzed and a higher growth rate for gallium arsenide compared to that of germanium was seen.

MeV gold irradiation induced damage in α-quartz: Competition between nuclear and electronic stopping

Abstract Damage creation in crystalline α-quartz by irradiation is studied using gold ions of energies between 0.5 and 10 MeV. For all ions, the total stopping power (d E /d x ) tot has a value of about 4.5 keV/nm, whereas the contribution of the electronic stopping power ranges from 0.93 keV/nm at 0.5 MeV to 3.6 keV/nm at 10 MeV. This variation allows us to test which role the nuclear and the electronic collisions plays for the damage processes. The kinetic of the ion induced damage was determined by channeling RBS and the volume increase by profilometry. Single ion impacts create damage when electronic stopping dominates, while several impacts are necessary to achieve damage in the nuclear stopping regime. A detailed analysis allows us to deduce the damage cross-sections of the two processes. The electronic stopping power of damage creation appears above an electronic d E /d x threshold of 1.4±0.3 keV/nm.