TY - JOUR
AU - Kondo, Yukihito
AB - Abstract The double-probe piezodriving specimen holder that was recently developed by some of the present authors is modified to introduce a laser irradiation port in one of its two arms. As a result, the new specimen holder consists of a piezodriving probe and a laser irradiation port, both of which can be three-dimensionally controlled by using piezoelectric elements and micrometers. While the piezodriving probe interacts with the specimen set in the holder in several ways, the laser beam causes photo-induced phenomena to occur. By performing electron holography using the new specimen holder, we demonstrate that it is possible to evaluate the change in the electric field resulting from the discharging effect of laser irradiation on organic photoconductors. electron holography, electric field, electric potential, photoconductor, photo-induced phenomenon, charging Recently, in addition to the improvement in the resolution limits of microscopes and spectroscopes, various modifications have been introduced in specimen holders for use in transmission electron microscopes (TEMs). For example, a piezodriving probe [1–5] and an environmental cell [6,7] have been developed, and in situ experiments have been performed extensively by utilizing such specimen holders. Some of the authors of this paper have also developed a double-probe piezodriving holder [8] and have used it for the conductivity measurement and electron holography analysis of an electric field under the electric shielding condition. In this paper, we briefly report the structure and performance of a new multifunctional TEM specimen holder; this holder contains a laser irradiation port in one of its two arms in addition to the piezodriving probe present in the older version of the double-probe piezodriving holder. This new specimen holder is expected to be useful for studying various photo-induced phenomena. As a preliminary application of this specimen holder, the electron holography technique [9,10] is used to demonstrate the change in the electric field because of the discharging effect of laser irradiation on organic photoconductors. In a previous paper [8], the structure of a double-probe piezodriving holder that was developed by some of the present authors was presented. In Fig. 1, the schematic illustration of a new specimen holder is shown. The double-probe piezodriving holder was modified to introduce a laser irradiation port in its left arm (arm 1). A light convergence function can be added at the top of the laser irradiation port. A plastic fiber is placed inside arm 1 in order to introduce the laser beam in the holder. Various laser systems with different energies can be placed outside the microscope. On the right-hand side of Fig. 1, an enlarged image of the top portion of the specimen holder is shown. The laser irradiation port and the probe can be manipulated independently in three dimensions by controlling the motions of arms 1 and 2, respectively. These two arms are driven by micrometers and piezoelectric elements, which are located in the tail portion of the holder. Three manual micrometers are used to effect the coarse movement of the laser irradiation port and the probe, while three piezoelectric elements are used to effect fine movement. In Fig. 2a, an image of the top portion of the specimen holder is shown. The specimen appears red when it is irradiated by a red laser beam, as shown in Fig. 2b. Fig. 1.  View largeDownload slide Schematic illustration of the new specimen holder equipped with a piezodriving probe and a laser irradiation port. On the right-hand side, an enlarged diagram of the top portion of the specimen holder is shown. Fig. 1.  View largeDownload slide Schematic illustration of the new specimen holder equipped with a piezodriving probe and a laser irradiation port. On the right-hand side, an enlarged diagram of the top portion of the specimen holder is shown. Fig. 2.  View largeDownload slide Images of the top portion of the specimen holder with the laser beam (a) off and (b) on. In (b), the red arrow indicates the direction of the laser beam. Fig. 2.  View largeDownload slide Images of the top portion of the specimen holder with the laser beam (a) off and (b) on. In (b), the red arrow indicates the direction of the laser beam. In this study, we used the specimen holder to investigate the discharging effect in commercial organic photoconductors, which are widely utilized in electrophotography. It should be noted that the discharging effect in organic photoconductors has not been directly analysed so far. In most laser printing systems, electrostatic latent charges are produced through the so-called corona discharge effect on the surface of the organic photoconductor, which comprises a charge-transport layer, a charge-generation layer, an undercoat layer and a substrate. In this study, instead of using the corona discharge effect to generate the electrostatic latent charges, frictional charging was employed along with a small glass rod, as shown in Fig. 3a. In order to avoid charging caused by the emission of electron-induced secondary electrons, we placed an electric shield above the specimen in a manner similar to that in a previous experiment on toner particles [11]. In order to shield the toner particles, a Mo plate was placed in one of the piezodriving arms. In the present study, a Mo plate was directly fixed at the top of the organic photoconductor, as shown in Fig. 3a: refer to ‘shield’ in the figure. After frictional charging using the glass rod, negative charges were expected to appear on the surface of the organic photoconductor, as shown in Fig. 3b. In order to investigate the formation of negative charges, electron holograms were observed, and reconstructed phase images were obtained, as shown in Fig. 4a. In the reconstruction process, the reference wave modulation caused by the electric field was neglected. In Fig. 4b, the electric potential distributions projected onto the incident electron beam along the line x–y are compared for two instants of time, 0 and 3 min. It should be noted that the electric potential produced by frictional charging does not change significantly for periods of time <3 min under the present experimental conditions. On the other hand, when the laser light is radiated onto the surface of the organic photoconductor, the electric potential around the organic photoconductor is observed to change considerably, as shown in Fig. 5a and b. It is likely that the electric potential distribution in Fig. 5a is not completely identical to that in Fig. 4a, although both of the observations represent the state after the frictional charging. Presumably, the deviation is due to differences in experimental conditions between those observations: e.g. position of the photoconductor with reference to the edge of the Mo shield, etc. In Fig. 6a, the change in the projected electric potential distribution before and after laser irradiation is shown. The irradiation time of the laser beam was 30 s, and the total time elapsed during the experiment, including the time of laser irradiation and the time taken for the hologram observation, was within 2 min. The change in the discharging effect with time is negligible according to the data in Fig. 4. In Fig. 6a, the electric field caused by the negative charges around the organic photoconductor is reduced because of laser irradiation (Fig. 3c). Some of the negative charges might have disappeared because of electron–hole annihilation, which involves the generation of holes in the charge-generation layer of the organic photoconductor as a result of laser irradiation, as shown in Fig. 6b. Thus, it has been demonstrated that the new specimen holder with a laser irradiation port is suitable for the investigation of photo-induced phenomena, such as the discharging effect of laser irradiation on organic photoconductors. Fig. 3.  View largeDownload slide (a) Schematic illustration of frictional charging by using a small glass rod. (b) Because of frictional charging, negative charges appear on the surface of the organic photoconductor. (c) The number of negative charges is reduced by the generation of electron-hole pairs in the charge-generation layer of the organic photoconductor. Fig. 3.  View largeDownload slide (a) Schematic illustration of frictional charging by using a small glass rod. (b) Because of frictional charging, negative charges appear on the surface of the organic photoconductor. (c) The number of negative charges is reduced by the generation of electron-hole pairs in the charge-generation layer of the organic photoconductor. Fig. 4.  View largeDownload slide (a) Reconstructed phase image showing the electric potential distribution after frictional charging. The phase is amplified by 3. (b) Electric potential distribution projected onto the incident electron beam along the line x–y for 0 and 3 min after frictional charging. In (b), for simplicity, the electric potential at the point ‘x’ is set ‘0’, since we are particularly interested in the change that is observed along the line x–y. Fig. 4.  View largeDownload slide (a) Reconstructed phase image showing the electric potential distribution after frictional charging. The phase is amplified by 3. (b) Electric potential distribution projected onto the incident electron beam along the line x–y for 0 and 3 min after frictional charging. In (b), for simplicity, the electric potential at the point ‘x’ is set ‘0’, since we are particularly interested in the change that is observed along the line x–y. Fig. 5.  View largeDownload slide Reconstructed phase images showing the electric potential distribution around the organic photoconductor (a) after frictional charging (before laser irradiation) and (b) after laser irradiation. In (a) and (b), the phase is amplified by 3. Fig. 5.  View largeDownload slide Reconstructed phase images showing the electric potential distribution around the organic photoconductor (a) after frictional charging (before laser irradiation) and (b) after laser irradiation. In (a) and (b), the phase is amplified by 3. Fig. 6.  View largeDownload slide (a) Change in the electric potential distribution, which is projected onto the incident electron beam, because of laser irradiation. The red and blue curves represent the electric potential that is observed along the red and blue lines in Fig. 5 (namely, line x–y), respectively. The electric potential at the point ‘x’ is set ‘0’, since we are particularly interested in the change that is observed along the line x–y. (b) Schematic illustration of the mechanism of negative charge reduction on organic photoconductors upon laser irradiation. CTL: charge-transport layer; CGL: charge-generation layer. Fig. 6.  View largeDownload slide (a) Change in the electric potential distribution, which is projected onto the incident electron beam, because of laser irradiation. The red and blue curves represent the electric potential that is observed along the red and blue lines in Fig. 5 (namely, line x–y), respectively. The electric potential at the point ‘x’ is set ‘0’, since we are particularly interested in the change that is observed along the line x–y. (b) Schematic illustration of the mechanism of negative charge reduction on organic photoconductors upon laser irradiation. CTL: charge-transport layer; CGL: charge-generation layer. In order to perform    further quantitative analysis, we need to consider the reference wave modulation caused by the electric field. The effect of the reference wave modulation can be incorporated as in the following manner. When the reference wave is modified by the electric field originating from the specimen, the observed phase shift obs(r) is modified as follows [11,12]:   (1)where the term (r+D) represents the influence of the perturbed reference wave, and D is a vector representing the width of the fringe overlap (i.e. the region which contains interference fringes) (see Reference [12] for details). For simplicity, we assume the presence of a point charge on the surface of the photoconductor (Fig. 7a). Consideration of Eq. (1) provides a calculated potential distribution near the point charge as shown in Fig. 7a. Note that the phase information is presented in terms of cos(obs) in Fig. 7, although the results in Figs 4 and 5 are given in terms of . It appears that the lower part in the simulation (Fig.7a) agrees with the observation (Fig. 7b) when the amount of point charge is assumed to be 3.3 × 10−17 C. More detailed studies about the charge distribution are under progress, which will be reported elsewhere. Fig. 7.  View largeDownload slide (a) Calculated potential distribution near a point charge that is placed on the surface of photoconductor. The effect of reference wave modulation is incorporated. The phase is amplified by 3, and the phase information is presented in terms of cos. (b) Observed potential distribution, which represents the state after frictional charging. The phase is amplified by 3. See the text for details. Fig. 7.  View largeDownload slide (a) Calculated potential distribution near a point charge that is placed on the surface of photoconductor. The effect of reference wave modulation is incorporated. The phase is amplified by 3, and the phase information is presented in terms of cos. (b) Observed potential distribution, which represents the state after frictional charging. The phase is amplified by 3. See the text for details. In conclusion, the new multifunctional TEM specimen holder equipped with a piezodriving probe and a laser irradiation port shows promise for use in the study of various photo-induced phenomena. Funding This study was mainly supported by a Grant-in-Aid for Scientific Research (S) (No. 19106002) from the Japan Society for the Promotion of Science. The authors also thank the Tohoku University Global COE Program “Materials Integration (International Center of Education and Research)” founded by MEXT, Japan, and the Post-Silicon Materials and Devices Research Alliance for the Special Education and Research Expenses for their support in some of the experiments. The authors thank Mr. H. Kawase, RICOH Ltd., for useful discussions on the organic photoconductor experiment. 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TI - Development of a multifunctional TEM specimen holder equipped with a piezodriving probe and a laser irradiation port
JF - Journal of Electron Microscopy
DO - 10.1093/jmicro/dfp018
DA - 2009-04-08
UR - https://www.deepdyve.com/lp/oxford-university-press/development-of-a-multifunctional-tem-specimen-holder-equipped-with-a-WuVjLPR09v
SP - 245
EP - 249
VL - 58
IS - 4
DP - DeepDyve
ER -