High-precision, large-domain three-dimensional manipulation of nano-materials for fabrication nanodevices
© Zou et al; licensee Springer. 2011
Received: 14 May 2011
Accepted: 27 July 2011
Published: 27 July 2011
Nanoscaled materials are attractive building blocks for hierarchical assembly of functional nanodevices, which exhibit diverse performances and simultaneous functions. We innovatively fabricated semiconductor nano-probes of tapered ZnS nanowires through melting and solidifying by electro-thermal process; and then, as-prepared nano-probes can manipulate nanomaterials including semiconductor/metal nanowires and nanoparticles through sufficiently electrostatic force to the desired location without structurally and functionally damage. With some advantages of high precision and large domain, we can move and position and interconnect individual nanowires for contracting nanodevices. Interestingly, by the manipulating technique, the nanodevice made of three vertically interconnecting nanowires, i.e., diode, was realized and showed an excellent electrical property. This technique may be useful to fabricate electronic devices based on the nanowires' moving, positioning, and interconnecting and may overcome fundamental limitations of conventional mechanical fabrication.
Keywordsnano-probe ZnS nanowire manipulation TEM-STM nanodevices
The main driving engine of the IT revolution has been geometrical miniaturization of transistors. This has been accomplished with a striking development in microfabrication technology, referred to as "Moore's law", i.e., the number of transistors on an integrated circuit (IC) doubles every 2 years, and industrial guidelines enable multiple devices to be integrated within a given chip area [1, 2]. For the past decade, however, the size of the microchips has remained roughly constant and we are approaching the atomic limit of a critical size. Clearly, this evolution cannot continue down this same path much longer. In reality, they are not likely to replace the ordinary transistors, but they may well provide the paradigm shift that will extend "Moore's law". One-dimensional nanoscaled materials, nanowires (NWs), nanotubes (NTs), or even composite nanowires made up of different materials represent attractive building blocks for hierarchical assembly of functional nanoscale devices, which can exhibit a real device with diverse performances and simultaneously function as the "wires", i.e., they can access and interconnect devices that could overcome fundamental limitations of conventional fabrication [3–7]. These unique properties and the intrinsically miniaturized dimensions of NWs' and NTs' building blocks may facilitate the continuation and extension of Moore's law and the evolutionary demand for even faster and smaller electronics in the future. So far, Ferry has developed so-called circuit cleverness by replacing field effect transistor (FET) of vertically oriented structures with parallel nanowires transistor ; it can be a great potential to create reconfigurable architectures in which the connections between different functional blocks are changed by switching just a few of the vertical transistors. Indeed, there has been considerable interest in multilayer electronics to offer a more efficient interconnection and processing of digital information. However, existing technologies for creating reconfigurable architectures are limited, such as chemical vapor deposition (CVD) process [9, 10], ion etching [11, 12], lithography [13–16], fluidic , and Langmuir-Blodgett technique , which are difficult with high-precision and large-domain three-dimensional (3D) manipulation of the nanowires for nanodevices fabrication with reconfigurable architectures. So, the developing new technique for achieving true 3D-integrated circuits based on the conventional complementary semiconductor transistor technology is critical and remains a challenge.
Herein, we develop a useful technique for high-precision, large-domain 3D manipulation of semiconductor nanowires for fabrication nanodevices, which was performed using a new scanning tunnel microscope (STM) - transmission electron microscope (TEM) holder commercialized by Nanofactory Instruments AB (Gothenburg, Sweden). We selected Sn-tipped, tapered ZnS semiconductor nanowires as a manipulative material, which were prepared via a Sn-catalyzed vapor-liquid-solid (VLS) growth process, as described in our previous reports. In our study, we innovatively fabricated semiconductor nano-probes of tapered ZnS nanowires through a melting and solidifying by electro-thermal process; and then the ZnS nanowires are manipulated by as-prepared nano-probes after a sufficiently electrostatic force is initiated between the probes and nanowires by direct current (DC) bias voltage, in which we can finely move individual nanowires at the desired location, with precisely controllable position and moving direction in 3D scale. With some advantages of high precision (with an accuracy of approximately 0.5 nm) and large domain (reaching 1 cm), the present manipulation technology could be used to realize top-down fabrication of nanostructure nanodevices made of three nanowires, which demonstrates good electricity properties and could be used in FET with vertically structure and other electronic devices. This present nanoscaled manipulation technology shows a new opportunity for fabrication nanodevices with a special structure, excellent properties, and potential applications.
ZnS, SnO, SnO2, and activated carbon powders were purchased from Sinopharm Chemical Reagent Co., Ltd. The tapered ZnS nanowires tipped with a spherical Sn particle were synthesized via a Sn catalyzed VLS growth process in a horizontal high-temperature resistance furnace. Briefly, a graphite crucible containing a mixture of ZnS (1.5 g), SnO (0.3 g), SnO2 (0.2 g), and activated carbon powders (0.1 g) was placed in the central zone of a quartz tube, which was heated to 1,150°C at a rate of 10°C min-1, kept at this temperature for 4 h, and then cooled to room temperature. The whole process was carried out under a constant flow of pure N2 at a rate of 450 mL min-1. Finally, the products were collected from the inner wall of the tube for our following STM-TEM manipulations.
Three-dimensional manipulation Study
The manipulations of as-prepared ZnS nanowires were carried out using a new STM-TEM holder commercialized by Nanofactory Instruments AB, which was arranged within a 200 kV field emission high-resolution TEM (HRTEM; JEM-2010F, JEOL Ltd., Tokyo, Japan). A gold (Au) cantilever was attached to a fixed electrical sensor, whereas a platinum (Pt) cantilever was placed on the piezo-movable side of the holder. At the beginning, relative positions between Pt and Au cantilevers were manually adjusted with tweezers under an optical microscope in order to obtain a minimal possible gap between them, which can be distinguished by eyes. Then, the X, Y, and Z positions of two cantilevers were adjusted through the nanoscale precision piezo-driven manipulator inside the TEM, so as to make horizontal between two cantilevers. A PC-compatible software automatically coordinates the final stages and controls the nanowire displacement and movement rate. All experiment processes are video recorded in real-time using a TV-rate charge-coupled device camera with a 30 frames per second recording speed. On the basis of the model adopted from the classical electricity, the electrical properties were evaluated by the dedicated software and electronics from Nanofactory Instruments AB.
Results and discussion
Here, let us consider the principle of the electro-thermal system of this ZnS nano-probe where the thicker and thinner ends of ZnS nanowire contact with the Sn particle and Au cantilever, respectively. When applying a bias voltage, a constant direct current I flows through this nano-probe system. The rate of heat quantity generated (per unit time) on the probe with a length ΔX is given by Q = I 2 ΔR. The term ΔR, the electrical resistance of the segment Δx, is given by ρΔx/ΔA, where ρ is the electrical resistivity of a given material, and ΔA is a cross-sectional area of the segment. Figure 2c schematically shows the tapered ZnS nanowire nano-probe inside the STM-TEM. The measured resistance of this nano-probe system, R m, can be expressed as: R m = R Pt + R Au + R NW, where the R Pt is the contact resistance between the Pt cantilever and Sn particle, R Au is the contact resistance between the Au cantilever and ZnS nanowire tip, and R NW is the intrinsic resistance of ZnS nanowire. In fact, the value of R Au becomes larger compared with that of R NW, because the area of the actual contact surface is considerably small; similarly, the value of R Pt is large compared with that of the Sn particle. Therefore, the corresponding values of Q Au and Q Pt around their contact regions are both larger than those on ZnS nanowire and Sn particle, giving rise to a local temperature increase. But, Sn particle is metallic conductor, with a lower resistance than that of ZnS nanowire, and Q Pt is lower than Q Au. For metals, Q is temperature dependent, and is usually given by Q = Q 0[1 + a(T - T 0)], where Q 0 is the electrical resistivity at a reference temperature T 0, and α is a temperature coefficient of the resistance and T is the temperature, which is normally positive for metals. The increase of local temperature at the contact region of the Sn particle leads to an increase of ΔR, and this further accelerates the temperature increase. Due to the high contact, electrical resistance (of the ZnS nanowire and Sn nanoparticle) with a low melting point (as discussed earlier) will make the Sn nanoparticle to be locally melted by a current. This will cause the Sn particle weld onto Pt cantilever and form a perfect physical contact between Pt cantilever and Sn particle. On the contrary, because ZnS nanowire has a high melting point (about 1,650-1,900°C) , welding process cannot occur at the contact region of the ZnS nanowire and Au cantilever, even if a local temperature increases at this region. After realization of this perfect physical contact, the electrical resistance at the contact region between Pt cantilever and Sn particle decreases, and then the amount of heat generated at the region decreases, and the local temperature at this region will drop. The local temperature at this region is so low that the contact region will be solidified, and the ZnS nanowire nano-probe is realized.
Statistics for the various positions of the ZnS nanowires
Number of attempts
Number of successful
Description of failure
Most importantly, semiconductor ZnS nanowire probe replacing common metal probe has been also successfully applied for manipulating low-melting point metals and ultrathin metal nanowires. Referring to welding of ultrathin Au nanowires, atomic diffusion and surface relaxation are obvious in the aforementioned nanoscale process [24–26]. It is well recognized that the diffusion barrier for a single metal atom on the ultrathin metal surface is quite low (typically less than 1 eV) . Thermal activation, even at room temperature, is enough to overcome such low barriers, so isolated metal atoms can diffuse rapidly by means of surface diffusion. The mechanical manipulation can clearly provide the necessarily extra driving force to facilitate the cold welding and unification of two nanowires. Additionally, because of low-melting point metals and ultrathin metal nanowires, current flowing through nanostructured metals faces a major problem that the nano-objects can be structurally and functionally damaged due to atom migration due to Joule heating and electromigration [28, 29]. Here, we innovatively have developed the nano-probes of semiconductor nanowires and excellently solved these mentioned problems.
In summary, we have fabricated a versatile nanoscaled probe with as-synthesized tapered ZnS nanowires with the STM-TEM holder through a melting and solidifying by an electro-thermal process. This fabricated ZnS nanowire probe can precisely, controllably, and 3D-scaled manipulate free-standing nanomaterials including semiconductor nanowires, metal nanowires, and nanoparticles, without structurally and functionally damage, to the desired locations for further making nanodevices. Interesting, by the developed manipulating technique, the nanodevices, i.e., vertical transistors, which were made of three vertically interconnecting nanowires were realized and showed an excellent electrical property, suggesting that it may be useful by the present technique to design and fabricate electronic devices based on nanowires' moving, positioning, and interconnecting. Importantly, this developed nanowire manipulation technology could overcome fundamental limitations of conventional mechanical fabrication and would bring a new opportunity for extending Moore's Law.
This work was supported from the National Natural Science Foundation of China (grant no. 50872020 and 50902021), the Program for New Century Excellent Talents of the University in China, the "Pujiang" Program of Shanghai Education Commission (grant no. 09PJ1400500), the "Dawn" Program of the Shanghai Education Commission (grant no. 08SG32), the Shanghai Leading Academic Discipline Project (grant no. B603), the Science and Technology Commission of Shanghai-based "Innovation Action Plan" Project (grant no. 10JC1400100), the "Chen Guang" project (grant no. 09CG27) supported by the Shanghai Municipal Education Commission and Shanghai Education Development Foundation, the Program for the Specially Appointed Professor by Donghua University (Shanghai, People's Republic of China), the Program of Introducing Talents of Discipline to Universities (no. 111-2-04), and the Program of Innovation Fund for Doctor Degree Dissertation (no. BC20101224).
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