Characterization of Phase Purity in Organic Semiconductors by Lattice‐Phonon Confocal Raman Mapping: Application to Pentacene

Abstract

propagation geometry, we have performed the retrieval [7,9] of the effective magnetic permeability l and the effective electric permittivity e on the basis of calculated optical spectra. Generally, the shape of the retrieved spectra of e and l are similar to those, e.g., shown in Figure 4 of our recent publication. [7] Thus, the retrieved spectra are not shown here. For the SRR parameters of the first row of Figure 4 we find a resonance in l with l < 0 around a wavelength of 2.4 lm. For the SRR parameters of the second row of Figure 4 we find a resonance in l around 1.7 lm with reduced oscillator strength, thus l > 0, but still with a negative magnetic susceptibility. The oscillator strength is yet further reduced for the SRR parameters of the third row of Figure 4, corresponding to a magnetic resonance around a wavelength of 1.2 lm.In conclusion, we have designed and realized a variety of different metamaterials, taking advantage of the rapid prototyping capabilities of FIB nanofabrication. In particular, we have demonstrated a continuous transition from squareshaped metallic pads that exhibit a twofold degenerate Mie (electric dipole) resonance to SRRs with a red-shifted fundamental magnetic-dipole response and one remaining Mie resonance. The retrieval of the corresponding calculated optical spectra reveals a magnetic response with a negative magnetic susceptibility (although l > 0) at a wavelength of 1.7 lm and a negative permeability (i.e., l < 0) at a wavelength of 2.4 lm for propagation in the split-ring array plane. Overall, our results show the robustness of the SRR concept. Such robustness is especially important for nanometer-sized SRR arrays where fabrication tolerances are much more of an issue than for microwave structures. Combined with metallic nanowires leading to a negative permittivity, our findings on magnetic split rings pave the road for left-handed metamaterials at telecommunication wavelengths. ExperimentalThe starting point of our nanofabrication process is a glass substrate, coated with a 5 nm thin film of indium tin oxide (ITO) and a 20 nm film of gold. The ITO acts as an adhesion promoter and enhances the quality of the gold film. Both films are deposited by electron-beam evaporation under high vacuum (10 -4 Pa). The FIB writing corresponds to an inverse process in the sense that the FIB removes material. We use a dual-beam FIB/SEM (scanning electron microscopy) system (Zeiss 1540 XB) and Ga + ions accelerated by a voltage of 30 kV. Typical FIB currents are 5 pA, typical exposure doses are 2900 lA s cm -2. Each SRR consists of a square with a side length of 280 nm in which a notch of depth u is cut. The arms of the resulting "U" have a typical width of 75 nm. All structures discussed here consist of an array of 35 × 35 SRRs with a lattice constant of 450 nm (total area of 16 lm × 16 lm). The exposure of one array like the one shown in Figure 2 takes approximately 20 min. Afterwards, the structure can be immediately inspected by the SEM of the dual-beam system and no further post-proce...

A Polymorph Lost and Found: The High‐Temperature Crystal Structure of Pentacene

“…X-ray powder diffraction and Raman scattering allow identification of the two polymorphic forms, [16] and pressure experiments have revealed that the 14.5 Å polymorph irreversibly transforms into the 14.1 Å polymorph at elevated pressures. [10,17,18] Energy-minimization using quasi-Monte-Carlo sampling have been employed to study the stability of polymorphs of pentacene: the two observed phases (14.1 Å and 14.5 Å) represent the deepest energy minima, [19] that is, the most stable forms of pentacene. Despite interest in the use of pentacene as an organicsemiconductor material, and the continuing efforts to characterize the various crystal forms, doubts have remained until now about both the existence of a high-temperature polymorph of pentacene and its structure and thermal-expansion properties.…”

“…Upon cooling, this phase could be observed down to room temperature, indicating quenching of this high-temperature phase, consistent with previous observations of polymorphism in pentacene powders. [18] Our first attempts to use powder diffraction to obtain a crystal structure of the high-temperature phase were hampered by the fact that no single-phase powder pattern could be obtained, and the peak overlap between the two phases was substantial. Further experiments using single crystals heated to temperatures above 463 K eventually provided sufficient data for a singlecrystal structure determination.…”

“…The type and density of structural defects is expected to strongly influence the kinetics of the phase transformation, which proves to be very sluggish in powders and results in large hysteresis. [18] In powders, where the number of defects is expected to be large, pentacene-LT and -HT coexist even at room temperature (with an approximate LT/HT ratio of 20:1 at 290 K), or far above the phase-transformation temperature (LT/HT = 1:20 at 563 K), with a gradual change of Table 1. Unit-cell parameters of pentacene-HT and -LT.…”

Raman Identification of Polymorphs in Pentacene Films

“…In order to optimize device performance, it is of course necessary to properly characterize the chemical and physical purity of the sample, as impurities and static or dynamic disorder have a decisive role in determining the carrier localization [3,4]. Pentacene is also the system on which the need of controlling the phase purity of the crystal has been first underlined, as different polymorphs may occur in the same crystallite [5]. The polymorphism in pentacene has indeed attracted many experimental and theoretical works [6][7][8][9][10].…”

Investigation on the laser ablation of SiC ceramics using micro‐Raman mapping technique

“…SiC ceramics have several kinds of crystal forms, and could be easily detected and distinguished in Raman spectra that provide a wide range of information about the chemical structures with very strong signal of frequency, intensity, and width [15][16][17], so that μ-RS has been used in detecting the composition changes of laser-ablated SiC surface [11,12]. Moreover, micro-Raman mapping technology, a spatially resolved technique, allows one to characterize the phase distribution of chemical structures in a specific region of material surface by 2D images [18][19][20], possibly an effective technique to study the laser ablation of SiC ceramics.…”