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Microstructure of strained La2CuO4+delta thin films on varied sub-strates

Microstructure of strained La2CuO4+δ thin films on varied substrates

Jiaqing He and Robert F. Klie

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA Gennady Logvenov and Ivan Bozovic

Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA

Yimei Zhu a)

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA Abstract

Layered perovskite La2CuO4+δ(LCO) thin films were epitaxially grown on SrTiO3 (STO) and LaSrAlO4 (LSAO) substrates by atomic-layer-by-layer molecular beam epitaxy. The lattice de-fects and residual strain in these films were investigated by means of transmission electron mi-croscopy and electron energy loss spectroscopy. The LCO films showed a high epitaxial quality with flat interfaces and top surfaces. Misfit dislocations with Burgers vector a<010> and shear defects were frequently observed at or near the film/substrate interfaces and in the films, respec-tively, for all the LCO films. In one LCO film, grown on STO at the highest temperature, 700 o C, was a two layered structure. The top layer was identified as rhombohedral LaCuO2 by electron energy loss spectroscopy combined with electron diffraction. In addition, stacking faults were observed in plane-view micrographs of one LCO film grown on the STO substrate. The residual strains were evaluated from the spacing of misfit dislocations at the film/substrate interface and

a) electronic mail: zhu@https://www.wendangku.net/doc/986283753.html,

from the split electron diffraction patterns. Possible mechanisms of strain relaxations are dis-cussed based on the observed defects.

PACS: 68.37.lp; 74.72.-h

1. Introduction

The epitaxial growth of La2CuO4+δ(LCO) superconductor films has attracted much attention because of the remarkable effects of epitaxial strain on the critical temperature, T c [1-15]. Basi-cally, substrates provide two types of in-plane strain for the films: compressive strain if the lat-tice constant of the film, a f, is greater than the lattice constant of the substrate, a s, and tensile strain if a f < a s (Figure 1). Usually, in-plane compressive strain raises T c, while in-plane tensile strain lowers it. Indeed, what is relevant is the residual strain that strongly depends on the lattice mismatch between the film and the substrate. The state of strain in a film can differ considerably from that found in bulk materials with respect to the configuration of microstructure and lattice defects. Thus, the electrical properties (e.g., T c) of a strained film also can differ from those in the bulk. The lattice mismatch between a thin film and a substrate is one of the important pa-rameters that greatly influence on the film’s structure, morphology, and its remaining strain. The energy stored within an epitaxial film due to the lattice misfit strain increases with the film’s thickness. At a certain critical thickness, t cr, it is energetically favorable to relieve the misfit strain by formation of dislocations at or near the substrate/film interface. Usually, the configura-tion of the defect and the residual strain depend not only on the lattice mismatch and the film thickness but also on the nature of substrate and on the processing conditions.

At room temperature, LCO is orthorhombic crystal (Cmca) with lattice parameters a0 = 0.53346 nm, b0 = 0.54148 nm, and c0 = 1.31172 nm [16]. LaSrAlO4 (LSAO) has a tetragonal structure with space group I4/mmm and lattice parameters a = 0.3755 nm and c = 1.262 nm [17]. SrTiO3 (STO) exhibits a typical cubic perovskite structure (Pm3m) with a lattice parameter a = 0.3905 nm. While the structure of LCO is similar to that of LSAO, one difference lies in the small ortho-

rhombic distortion (about 0.5%) of the LCO lattice. For convenience in comparison with LSAO and STO, in the following discussion, we regard the LCO lattice as a pseudo-tetragonal one with lattice parameters a = (a02+b02 )1/2/2 = 0.38005 nm and c = c0 = 1.31172 nm. The lattice mis-match is by given by

f =

f s

f a a-

(1)

where a f and a s are the lattice parameter of the film and the substrate, respectively. From the lat-tice parameters listed above, the lattice mismatch is +2% for LCO /LSAO and -2.75% for LCO /STO. The mismatch with positive sign can induce an in-plane compressive stress in the film while that with negative sign produces an in-plane tensile stress.

In this paper, we report investigation of the microstructure and remaining strains in LCO films grown on STO and LSAO substrates, by means of transmission electron microscopy (TEM) combined with electron energy loss spectroscopy. We focus on the possible effects of the sub-strate, microstructure and defect configuration on the level of strain in the films.

2. Experiments

The films used in this study were all grown using the atomic-layer-by-layer molecular beam epi-taxy (ALL-MBE) technique, which has been described in detail previously [18,19]. LCO films were synthesized in a unique multi-chamber molecular beam epitaxy system, designed to enable atomic-layer engineering and optimization of complex oxide materials. The growth chamber contains 16 metal sources and a source of pure ozone. Deposition rates are mapped using a scan-ning quartz-crystal oscillator, and they are controlled accurately in real time by a 16-channel

atomic absorption spectroscopy system. The film’s growth is monitored by a reflection high-energy electron diffraction (RHEED) system and other state-of-the art surface analysis tools [19]. The films were also characterized in detail by atomic-force microscopy (AFM) and x-ray diffraction (XRD). Here, we describe the microstructure of three samples: A is a 50- nm-thick LCO film deposited on LSAO at T s≈ 650 0C, samples B and C are 30-nm-thick LCO films on STO deposited at T s≈ 650 0C and T s≈ 700 0C, respectively. TEM and HRTEM investigations were carried out in JEOL -3000F and -4000EX microscopes. The compositional homogeneity of the LCO thin film was investigated by electron energy loss spectroscopy (EELS). High-resolution image numerical simulations were carried out with the MacTempas computer program with the following parameters as input: spherical aberration of 1mm, defocus spread of 8 nm, semiconvergence angle of illumination of 0.55mrad, and diameter of the objective lens aperture of 7 nm.

3. Results

Low-magnification lattice images of samples A, B, and C, taken with the incident electron beam parallel to the [110] zone axis of the films, are shown in Figs.2 (a)–(c), respectively. Very flat top surfaces and interfaces are seen in all the films. The horizontal arrowheads in the images denote the interfaces between the thin films and the substrates. The epitaxial nature of the 50- nm-thick LCO film for sample A and 30-nm-thick films for samples B and C are evident from Fig. 2. The films A and B are homogeneous, while the film C has two layers, with the LaCuO2 layer on top, as discussed below.

Figure 3 shows selected-area electron diffraction (SAED) patterns for three samples along the [110] LCO zone axis acquired with an aperture that simultaneously collects scattered electrons coming from the LCO films, and from the LSAO or STO substrates. The orientation relation-ships between the LCO film and the LSAO substrate can be described as [001]LCO || [001]LSAO and [110]LCO || [010]LSAO, as shown in Fig. 3(a). The SAED pattern from the LCO/STO film in Figs. 3(b) and (c) clearly demonstrate similar orientation relationships, viz. [001]LCO || [001]STO and [110]LCO || [010]STO. In Fig. 3, some distinct splits of diffracted spots also can be detected along both the out-of-the-plane and the in-plane direction. Image calibration allows us to deter-mine the lattice parameters of the LCO films and top layer in sample C (given in Table 1), as-suming a tetragonal LSAO lattice with a = 0.3754 nm and c= 1.2635 nm, see Fig. 3 (a), and the cubic STO lattice with a STO = 0.3905 nm, see Fig.3 (b)-(c).

The lattice parameters of the three samples, A, B and C in Table 1 indicate that massive relaxa-tion mechanisms exist within all of them although some residual strains remain in the LCO films that are under in-plane compressive strain (in the sample A) or under in-plane tensile strain (in samples B and C). Such a lattice misfit relaxation is accommodated through several mechanisms, i.e., via formation of: (i) a network of misfit dislocations at or near the interfaces (Fig.4), often considered as the commonest mechanism; (ii) planar defects such as shear defects (Fig. 5) and stacking faults (Fig. 7), and, (iii) possible secondary phase LaCuO2 (Fig. 6).

It is well known that lattice mismatch can be accommodated either by straining the lattice or by the formation of misfit dislocations at the interface depending on the elastic properties of the materials and the film’s thickness. To explore this accommodation, the interfaces of all the films

were investigated by HRTEM. For the LCO film on the LSAO substrate, the film-substrate inter-face appears atomically sharp; over large areas only a few dislocations can be detected at or close to the LCO/LSAO interface due to small mismatch between LCO film and LSAO substrate. Fig-ure 4 (a) shows the [110] lattice fringe image of the LCO film on the LSAO. One misfit disloca-tion with a Burgers vector a<010> in the film exhibits a stand-off of 5 nm from the interface be-tween the LCO film and LSAO substrate due to the different elastic properties between LCO film and LSAO substrate [20] or to local composition fluctuation [18,19]. For LCO films on STO, numerous dislocations are observed at the LCO/STO interface. Figure 4(b) shows a lattice image of an LCO film on STO (sample B). There are two observable misfit dislocations (denoted by vertical arrows at the interface of the LCO film and the STO substrate) with Burgers vector of a[010], according to the Burgers circuit analysis surrounding the dislocation core (see Fig.4).

The possible residual mismatch strain in LCO/STO film was assessed by consideration of the separation distance and the Burgers vector of these misfit dislocations. As measured over a long distance along the interface we determined an averaged value of 14.6 nm for the separation dis-tance between two dislocations with the Burgers vector a<100>. Assuming that the misfit strain in the film is completely released by generation of misfit dislocations, the separation distance S of the misfit dislocations can be calculated from the following equation:

S = b/f = bd f / (d s-d f) (2)

where b is the magnitude of the Burgers vector of misfit dislocations in the interface plane, while d f and d s are the bulk values of inter-planar distances of the film material and the substrate, re-spectively. Inserting the value of the Burgers vector b = 0.39 nm, and using d f = 0.38 nm, d s =

0.39 nm, we obtained a dislocation spacing of 14.2 nm. The experimental value, 14.6 nm, is therefore almost equal to the value expected for a completely relaxed system. This result, corre-sponding to that in Fig. 2(c), suggests that the mismatch strain is almost completely relieved in the film.

Figure 5 (a) shows one type of planar defects, an octahedral edge-shear defect, in an LCO film that originates at the interface between the LCO film and the STO substrate due to non-stoichiometry. The two domains are translated by 1/6 of the unit cell dimension along the (031) plane, R= 1/6[031]LCO. Figure 5 (b) is a possible model for such a shear defect, showing an ex-cess of oxygen atoms and possibly also of La cations. The simulation high-resolution image of this modeling is inserted in Figure 5(a); it revealed a good fit with the lattice images. The possi-ble mechanisms of formation of shear defects are lattice mismatch strain [23], steps [15], or stacking faults [12]. However, neither steps nor stacking faults are seen in Fig. 5(a), and there-fore, the shear defects also may contribute to the relaxation of the misfit strain.

Figure 6 (a) is a cross-section image of a LCO film, taken along the [100] zone axis of the STO substrate that clearly shows two different structural layers distribution. The interface between the two layers is quite rough due to the variation in thickness of the top layer (from 10 nm to 25 nm away from the LCO/STO interface), however, the top surface is flat. To determine the film’s chemical composition, a line scan of EELS measurement was performed from the STO substrate, via the LCO film and the secondary-phase layer, to the glue layer, as marked by A-B in Fig. 6 (a) [24]. The energy range from 450 eV to 1000 eV was monitored, which included the O K-, La M4,

-, and Cu L2, 3-edges. Figure 6 (b) shows the EELS spectra obtained from these four typical ar-5

eas (substrate, film, secondary phase and glue) in the scanning transmission electron microscopy (STEM) mode with a probe size 1 nm in diameter. In these spectra, the energy resolution is about 1.2 eV for the FWHM (full width at half maximum) of the zero-loss peak. We note that the in-tensity of the La peak in spectra 3 is much less than that in spectra 2, indicating an La deficiency in the secondary phase. Figure 6 (c) presents the line scanning EELS results after background subtraction and removal of the thickness effect. It reveals show how the composition of the sam-ple (the La-, Cu- and O- content) varies with scanning position. Clearly, the top layer still con-tains all three elements, La, Cu and O, but the amount of La is about half of that in the LCO film. Taking into account the lattice parameter obtained from the electron diffraction pattern in Fig. 6 (c), we conclude that the top layer is LaCuO2, i.e., rhombohedral with a group space of R3m and the lattice parameters a = b = 0.38 nm and c = 1.70 nm.

Figure 7 shows plane-view micrographs of a wormlike meandering defect in the LCO film on the STO substrate. Two enlarged lattice images of the marked areas, taken along the [001] direction, indicate that the defect is a stacking fault rather than a misfit dislocation. The surface of STO substrates often shows steps, terraces and kinks, due to miscuts, that strongly influence on the microstructure of the LCO films. Hence, these wormlike stacking faults are most likely formed as the consequence of the specific surface structure of STO and the growth mode of the LCO films.

4．Discussion

The above results show that the quality and perfection of LCO films strongly depend on the sub-strates with different lattice mismatch to the film material. The effects of substrate extend not

only to generation of misfit dislocations but also of planar defects and formation of secondary phase(s). For a system that exhibits a lattice mismatch, one can define [25] the film’s critical thickness, t cr , can be defined as

t cr = )ln()1(8)cos 1(2b

T f b cr βνπθν+? (3) Here, b is the magnitude of the Burgers vector of the misfit dislocation in terms of the substrate lattice, θ is the angle between the Burgers vector and the dislocation line of the misfit disloca-tion, f is the misfit, ν is the average Poisson ratio of the film and the substrate, and β is the cut-off radius of the dislocation core with β≈4 [26]. To calculate the critical thickness in c -axis-oriented single-crystal films, the misfit segments of half-loops were assumed to be edge disloca-tions with Burgers vectors of a[010] type, gliding on the {010} plane. Using the parameters b = 0.3905 nm, f = 0.0275, ν = 0.3 and θ = 900, we calculated the critical thickness for STO substrate to be t cr ≈ 1.02 nm.

If the thickness of the film is less than the critical thickness t cr , the mismatch between the sub-strate and the film is accommodated by elastic lattice strain, i.e., no crystal defects such as misfit dislocations will be formed. When the film’s thickness exceeds t cr , the formation of misfit dislo-cations is energetically more favorable than the elastic strain accommodation [25]. The critical thickness depends on the level of lattice mismatch and on the elastic properties of the two mate-rials. Here we found the strain to be almost completely relaxed by forming defects, since all three films are much thicker than the t cr . We also note that the superconducting transition (critical) temperature was close to the bulk’s value. The high perfection of the LCO film on LSAO is based on the very small lattice mismatch in this system; nevertheless some misfit dislocations

were found here as well at or near the interface because the critical thickness for LCO films on LSAO is smaller than the film thickness, d = 30 nm. According to the measurements of spacing and Burgers vectors of the misfit dislocations, we also would expect no residual strain in LCO films on STO.

We noted that in sample C there was a LaCuO2 secondary phase in LCO film. Similar observa-tion (LaCuO3-δ at the interface between LCO films and STO substrate) was reported by Houben [15]. Kong et al. [11] also observed a secondary phase at the interface between a La2CuO4F x film and an STO substrate. In those studies, precipitates were detected close to the interface between the film and the substrate. In the present study, we have observed the secondary phase LaCuO2 at the top surface of LCO films. The formation of LaCuO2 precipitates in the film matrix could originate from local fluctuations in chemical composition and possibly from different surface mobility of constituent atoms. Since the lattice parameters of LaCuO2 are different from those of LCO, the formation of precipitates also may contribute to partially relieving the misfit strain that is incompletely accommodated by misfit dislocations.

5. Conclusions

The microstructure and residual strain of LCO films grown on STO and LSAO substrates by ALL-MBE technique were investigated at atomic scale by means of TEM, combined with HRTEM, electron diffraction and EELS analysis. The strain in LCO films on both types of sub-strates was found to be mostly relaxed by the formation of misfit dislocation cores, although the small lattice mismatch was accommodated by elastic strain in the LCO film on the LSAO sub-strate. In all the films, we observed planar defects (shear defects and stacking faults) as well as

one type of misfit dislocations (with the Burgers vectors a<010>) in the interface area between the film and the substrate. A secondary-phase formation was also facilitated strain relaxation.

Acknowledgment

This work was supported by the U.S. Department of Energy, Division of Materials, Office of Basic Energy Science, under Contract No. DE-AC02-98CH10886.

Table 1

The lattice parameters of LCO films on LSAO and STO substrates determined from super-imposed electron diffraction patterns with reference to the reflection spots of LSAO or STO. Parameter Sample Growth

temperature

(o C) substrate Film thickness (nm)

a LCO (nm) c LCO (nm) Bulk _ _ _ 0.38005 1.31172

A 650 LSAO 50 0.3798 1.3265

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Figure Captions

Fig. 1. Two different types of in-plane strain in LCO films on various substrates.

Fig. 2 a-c. Low-magnification cross-section TEM images of samples A, B and C, respec-tively. In all the samples, there are sharp and flat top surfaces and interfaces between films and substrates are observed. A secondary phase is visible close to the top surface in sample C.

Fig. 3. SAED patterns taken along (a) the [110] LSAO and (b) and (c) the [100] STO zone axes. They result from simultaneous diffraction coming from the films and from parts of the substrates. Some distinct splits spots are detected along both the out-of-plane and the in-plane directions in (a) and (b); besides these, some extra diffraction spots are observed in (c) and its partially enlarged image (bottom right).

Fig. 4. Enlarged lattice images showing the sharp interfaces between films and substrates. (a) A misfit dislocation near the interface between LCO and LSAO; (b) two misfit dislocations with Burgers vectors a[010] at the interface of LCO and STO. The positions of dislocations are marked by the arrows.

Fig. 5. (a) An octahedral corner-shearing defect with displacement vector R = 1/6[031]; and (b) a possible model of the shear defect, with the triangle symbols representing La cation va-cancies. The simulated lattice image of this model is shown in insert (a)

Fig.6. (a) Cross-section lattice images of an LCO film on STO substrate. A secondary phase with structure different from LCO is delineated by the dotted curve lines. (b) EELS spectra in the energy range from 450 eV to 950 eV obtained from the four typical areas in Fig. 6(a).

(c) The compositional distribution (La, Cu and O) obtained from the line scan of EELS spec-tra from the regions of substrate STO, the LCO film, the secondary phase (S) and the glue in Fig. 6(a). The horizontal axis is the distance (in nm) from the interface between LCO film and STO substrate.

Fig.7. A worm-like meandering planar defect from plane-view low-magnification image of an LCO film on STO with two enlarged lattice images (circled areas) showing lattice mis-match due to stacking faults.

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21、薄、搏、博、膊、傅、缚、簿： 22、铺、捕、哺、埔、甫、辅、圃、匍、蒲、 23、满、瞒、蹒、 24、景、影： 25、采、彩、睬、踩： 26、尚、淌、倘、躺、趟： 27、每、海、梅、悔、诲、霉： 28、朋、棚、鹏、 29、谁、唯、维、推、雕、难、雅、崔、摧、催、璀、稚、雏、 30、替、潜 31、富、福、幅、辐、蝠、副 32、鬼、槐、愧、魂、魄、魔 33、车、军、浑、挥、晖、辉 34、央、奂、涣、唤、换、焕、映、英、 35、娄、屡、缕、偻、搂、数、喽、 36、昆、混、棍 37、周、凋、调、绸、稠、惆、碉、雕、 38、曼、漫、慢、蔓、谩、幔、馒、 39、兹、慈、磁、滋 40、莫、漠、寞、摸、模、膜、 41、暮、蓦、墓、幕、慕、摹 42、象、像、橡、

43、樱、婴、鹦 44、告、浩、皓、靠、 45、牙、呀、芽、雅、讶 46、漆、膝 47、勿、忽、匆、葱 48、繁、敏、 49、累、螺、 50、亭、停、婷 51、曾、赠、憎、僧 52、班、斑 53、直、值、植、殖、 54、具、俱、惧、飓、 55、真、镇 56、正、证、症、政、征 57、珍、诊、 58、留、溜、榴、榴 59、揭、竭、 60、旦、担、坦 61、摇、谣、遥、瑶 62、非、韭、徘、辈、悲、斐、裴、靠、扉、霏、菲、匪、蜚、排 63、荒、谎、慌、 64、旬、询、殉

关于时间管理的英语作文 manage time

How to manage time Time treats everyone fairly that we all have 24 hours per day. Some of us are capable to make good use of time while some find it hard to do so. Knowing how to manage them is essential in our life. Take myself as an example. When I was still a senior high student, I was fully occupied with my studies. Therefore, I hardly had spare time to have fun or develop my hobbies. But things were changed after I entered university. I got more free time than ever before. But ironically, I found it difficult to adjust this kind of brand-new school life and there was no such thing called time management on my mind. It was not until the second year that I realized I had wasted my whole year doing nothing. I could have taken up a Spanish course. I could have read ten books about the stories of successful people. I could have applied for a part-time job to earn some working experiences. B ut I didn’t spend my time on any of them. I felt guilty whenever I looked back to the moments that I just sat around doing nothing. It’s said that better late than never. At least I had the consciousness that I should stop wasting my time. Making up my mind is the first step for me to learn to manage my time. Next, I wrote a timetable, setting some targets that I had to finish each day. For instance, on Monday, I must read two pieces of news and review all the lessons that I have learnt on that day. By the way, the daily plan that I made was flexible. If there’s something unexpected that I had to finish first, I would reduce the time for resting or delay my target to the next day. Also, I would try to achieve those targets ahead of time that I planed so that I could reserve some more time to relax or do something out of my plan. At the beginning, it’s kind of difficult to s tick to the plan. But as time went by, having a plan for time in advance became a part of my life. At the same time, I gradually became a well-organized person. Now I’ve grasped the time management skill and I’m able to use my time efficiently.

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6、很多时候选择和被选择是很矛盾的，正如你选不选择我的回答一样！ 7、在一样事和另一样事中，你必须做出选择。 8、选择你喜欢的，喜欢你选择的。 9、星期天，我选择去敬老院，帮助老太太做卫生。 10、对于你提的这个问题，我选择回答。 11、如果你选择了一个目标，就要坚持不懈，努力争取！ 12、既然你先择了，就不要后悔。 13、作为父母，我们不想干预你的选择，我们只想给你提点建议。 14、即然你选择了这个职业，你就要对你的选择负责。 15、他们之间你只能选择一个。

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Dear teacher and colleagues: my topic is on “spare time”. It is a huge blessing that we can work 996. Jack Ma said at an Ali's internal communication activity, That means we should work at 9am to 9pm, 6 days a week .I question the entire premise of this piece. but I'm always interested in hearing what successful and especially rich people come up with time .So I finally found out Jack Ma also had said :”i f you don’t put out more time and energy than others ,how can you achieve the success you want? If you do not do 996 when you are young ,when will you ?”I quite agree with the idea that young people should fight for success .But there are a lot of survival activities to do in a day ,I want to focus on how much time they take from us and what can we do with the rest of the time. As all we known ,There are 168 hours in a week .We sleep roughly seven-and-a-half and eight hours a day .so around 56 hours a week . maybe it is slightly different for someone . We do our personal things like eating and bathing and maybe looking after kids -about three hours a day .so around 21 hours a week .And if you are working a full time job ,so 40 hours a week , Oh! Maybe it is impossible for us at

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关于人英语演讲稿(精选多篇) 关于人的优美句子 1、“黑皮小子”是我对在公交车上偶遇两次的一个男孩的称呼代号。一听这个外号，你也定会知道他极黑了。他的脸总是黑黑的；裸露在短袖外的胳膊也是黑黑的；就连两只有厚厚耳垂的耳朵也那么黑黑的，时不时像黑色的猎犬竖起来倾听着什么；黑黑的扁鼻子时不时地深呼吸着，像是在警觉地嗅着什么异样的味道。 2、我不知道，如何诠释我的母亲，因为母亲淡淡的生活中却常常跳动着不一样的间最无私、最伟大、最崇高的爱，莫过于母爱。无私，因为她的爱只有付出，无需回报；伟大，因为她的爱寓于

普通、平凡和简单之中；崇高，是因为她的爱是用生命化作乳汁，哺育着我，使我的生命得以延续，得以蓬勃，得以灿烂。 3、我的左撇子伙伴是用左手写字的，就像我们的右手一样挥洒自如。在日常生活中，曾见过用左手拿筷子的，也有像超级林丹用左手打羽毛球的，但很少碰见用左手写字的。中国汉字笔画笔顺是左起右收，适合用右手写字。但我的左撇子伙伴写字是右起左收的，像鸡爪一样迈出田字格，左看右看，上看下看，每一个字都很难看。平时考试时间终了，他总是做不完试卷。于是老师就跟家长商量，决定让他左改右写。经过老师引导，家长配合，他自己刻苦练字，考试能够提前完成了。现在他的字像他本人一样阳光、帅气。 4、老师，他们是辛勤的园丁，帮助着那些幼苗茁壮成长。他们不怕辛苦地给我们改厚厚一叠的作业，给我们上课。一步一步一点一点地给我们知识。虽然

他们有时也会批评一些人，但是他们的批评是对我们有帮助的，我们也要理解他们。那些学习差的同学，老师会逐一地耐心教导，使他们的学习突飞猛进。使他们的耐心教导培养出了一批批优秀的人才。他们不怕辛苦、不怕劳累地教育着我们这些幼苗，难道不是美吗？ 5、我有一个表妹，还不到十岁，她那圆圆的小脸蛋儿，粉白中透着粉红，她的头发很浓密，而且好像马鬓毛一样的粗硬，但还是保留着孩子一样的蓬乱的美，卷曲的环绕着她那小小的耳朵。说起她，她可是一个古灵精怪的小女孩。 6、黑皮小子是一个善良的人，他要跟所有见过的人成为最好的朋友！这样人人都是他的好朋友，那么人人都是好友一样坦诚、关爱相交，这样人与人自然会和谐起来，少了许多争执了。 7、有人说，老师是土壤，把知识化作养分，传授给祖国的花朵，让他们茁壮成长。亦有人说，老师是一座知识的桥梁，把我们带进奇妙的科学世界，让