REPORTS ng loops of the nanobelt, as in a childs"Slinky" orientation. The coiling of the nanobelt introduc- The loose end of the nanobelt in Fig. 2F oring(7), by interfacing its(0001)-Zn and es a small helical angle of -0.3 all has a(0001) planar defect located close to the (0001)o planes at the same crystallographic rotation is hardly detectable by the el middle of its width, which suggests that the planar defect was produced during the nano- ig. 3. Structure of the A belts growth and is the key for producing the fastest growth of the nanobelt along its axial direction(8, 9). We counted 33 coiling loops and B)Bright-field and in the dark-field TEM displayed in ark-field TEM images corded from the na Fig. 2H. HRTEM indicates that besides the oring, with the incident planar defect inside the nanobelt, a stacking fault is formed at the interface between the to the ring plane.(C)ED hich is introduced pattern recorded from he lattices of the zn-terminated and o-ter he nanoring. The pat 01111101 minated(0001) polar surfaces(10). The in- tern shows vertical mir- ror symmetry, and the terface between the loops is coherent, epi- extra diffraction spots at taxial, and chemically bonded(Fig. 21) the two sides are from The type Il nanoring structure is presented the cylindrical bending. Fig. 3B) TEM images show that the complete nanoring is a single crystal, which again implies recorded from the tral symmetric line in A).(E)Enlarged TEM single-crystal ribbon around the circumference. 8 that the nanoring shell is a uniformly deformed, Electron diffraction(Fig 3C)and the corre- a marked in(A), showing entral region(Fig. 3D) show that the radial total number of loops direction of the nanoring is [1213, the tangen- o 100.(F)Dark-field TEM the nanoring after it was is no dislocation in the volume. Figure 3E a tilted by displays the enlarged view of the comer indi- cated in Fig. 3A, which shows an end of the nanobelt(indicated by an arrowhead) and the 8 of Zno and the A rew coiling of the nanobelt. The pitch dis. a tance for the coiling is 10 nm, and the total S anes discussed in the number of loops is 100. The contrast produced by stacking faults parallel to the nanoring +(0001)polar surfaces. plane is visible, but the(0001) stacking fau growth process and cor- plane is at an angle of -28 from the nanor- 5 -15 nm thick, as clearly shown by tilting the a initiation and formation nanoring by 15%(Fig. 3F) The growth of the nanoring structures can be a polar nanobelt. The na- understood on the basis of the polar surfaces of noring is initiated by the ZnO nanobelt. The wurtzite-structured ZnO folding a nanobelt into a crystal is described schematically as a number of op with overlapped (1210 1 um alternating planes composed of tetrahedral coor- ds driven by long dinated 0-and Zn2+ ions, stacked alternatively D along the c axis (Fig. 4A). The oppositely among the polar charged ions produce positively charged(0001)- chemical bonding stabi- Zn and negatively charged(0001)-0 polar sur- lizes the coiled ring faces. The polar nanobelt, which is the building structure and the block of the nanoring, grows along [1010, with aneous self-coiling of side surfaces *(1210) and top/bottom surfaces the nanobelt is driven by 8 +(0001)(4), and has a typical width of 15 nm the energ contributed by pola and thickness of -10 nm(). The planar defect parallel to(0001)lowers the nanobelt nd elastic deformation. is key to producing the fastest anisotrop Calculated energy along [1010), but it does not affect polarity of the nanobelt. Therefore, the nanobelt AEBatm)before anda 7008009001000 er folding of a straight Ring Diameter(nm) has polar charges on its top and bottom surfaces (Fig. 4B). If the surface charges are uncompen p-structured nanoring as a function of the ring radius and the number of loops. Nanobelt width= 20 ated for during growth(In), the nanobelt may thickness =16 nm, Youngs modulus= 50 GPa, and surface charge density lol=0.057 C/m2.The end to fold itself as it lengthens, in order to calculation gives the threshold radius under which initiation of the nanoring structure is energetically minimize the area of the polar surface. One unfavorable. The smallest nanoring observed has D=0.8 um. possible way to reduce the electrostatic energy is 1350 27FebRuaRy2004Vol303ScieNcewww.sciencemag.orging loops of the nanobelt, as in a child’s “Slinky” spring (7), by interfacing its (0001)-Zn and (0001¯)-O planes at the same crystallographic orientation. The coiling of the nanobelt introduc￾es a small helical angle of 0.3°; this small rotation is hardly detectable by the ED pattern. The loose end of the nanobelt in Fig. 2F has a (0001) planar defect located close to the middle of its width, which suggests that the planar defect was produced during the nano￾belt’s growth and is the key for producing the fastest growth of the nanobelt along its axial direction (8, 9). We counted 33 coiling loops in the dark-field TEM image displayed in Fig. 2H. HRTEM indicates that, besides the planar defect inside the nanobelt, a stacking fault is formed at the interface between the adjacent loops, which is introduced to match the lattices of the Zn-terminated and O-ter￾minated (0001) polar surfaces (10). The in￾terface between the loops is coherent, epi￾taxial, and chemically bonded (Fig. 2I). The type II nanoring structure is presented in Fig. 3. Bright-field (Fig. 3A) and dark-field (Fig. 3B) TEM images show that the complete nanoring is a single crystal, which again implies that the nanoring shell is a uniformly deformed, single-crystal ribbon around the circumference. Electron diffraction (Fig. 3C) and the corre￾sponding HRTEM image recorded from the central region (Fig. 3D) show that the radial direction of the nanoring is [12¯13¯], the tangen￾tial direction is [101¯0], and the nanoring plane is (12¯12) (see the model in Fig. 4A), and there is no dislocation in the volume. Figure 3E displays the enlarged view of the corner indi￾cated in Fig. 3A, which shows an end of the nanobelt (indicated by an arrowhead) and the screw coiling of the nanobelt. The pitch dis￾tance for the coiling is 10 nm, and the total number of loops is 100. The contrast produced by stacking faults parallel to the nanoring plane is visible, but the (0001) stacking fault plane is at an angle of 28° from the nanor￾ing axis. The nanoring has a thin crystal wall 15 nm thick, as clearly shown by tilting the nanoring by 15° (Fig. 3F). The growth of the nanoring structures can be understood on the basis of the polar surfaces of the ZnO nanobelt. The wurtzite-structured ZnO crystal is described schematically as a number of alternating planes composed of tetrahedral coor￾dinated O2– and Zn2 ions, stacked alternatively along the c axis (Fig. 4A). The oppositely charged ions produce positively charged (0001)- Zn and negatively charged (0001¯)-O polar sur￾faces. The polar nanobelt, which is the building block of the nanoring, grows along [101¯0], with side surfaces (12¯10) and top/bottom surfaces (0001) (4), and has a typical width of 15 nm and thickness of 10 nm (7). The planar defect parallel to (0001) lowers the nanobelt energy and is key to producing the fastest anisotropic growth along [101¯0], but it does not affect the intrinsic polarity of the nanobelt. Therefore, the nanobelt has polar charges on its top and bottom surfaces (Fig. 4B). If the surface charges are uncompen￾sated for during growth (11), the nanobelt may tend to fold itself as it lengthens, in order to minimize the area of the polar surface. One possible way to reduce the electrostatic energy is Fig.3. Structure of the type II ZnO single￾crystal nanoring. (A and B) Bright-field and dark-field TEM images recorded from the na￾noring, with the incident electron beam parallel to the ring plane. (C) ED pattern recorded from the nanoring. The pat￾tern shows vertical mir￾ror symmetry, and the extra diffraction spots at the two sides are from the cylindrical bending of the single-crystal rib￾bon. (D) HRTEM image recorded from the cen￾tral symmetric line in (A). (E) Enlarged TEM images from area e marked in (A), showing the coiling layers. The total number of loops forming this nanoring is 100. (F) Dark-field TEM image recorded from the nanoring after it was tilted by 15°. Fig.4. (A) Structure model of ZnO and the corresponding crystal planes discussed in the text, showing the (0001) polar surfaces. (B to D) Proposed growth process and cor￾responding experimen￾tal results showing the initiation and formation of the single-crystal na￾noring via self-coiling of a polar nanobelt. The na￾noring is initiated by folding a nanobelt into a loop with overlapped ends driven by long￾range electrostatic inter￾actions among the polar charges. Short-range chemical bonding stabi￾lizes the coiled ring structure, and the spon￾taneous self-coiling of the nanobelt is driven by minimizing the energy contributed by polar charges, surface area, and elastic deformation. (E) Calculated energy gain (E  EDeform EElectro) before and af￾ter folding of a straight polar nanobelt into a loop-structured nanoring as a function of the ring radius and the number of loops. Nanobelt width  20 nm, thickness  16 nm, Young’s modulus  50 GPa, and surface charge density  0.057 C/m2 . The calculation gives the threshold radius under which initiation of the nanoring structure is energetically unfavorable. The smallest nanoring observed has D  0.8 m. R EPORTS 1350 27 FEBRUARY 2004 VOL303 SCIENCE www.sciencemag.org on December 19, 2006 www.sciencemag.org Downloaded from