**2. Fabrication and evaluation method of QC devices**

#### **2.1 Fabrication method of QC devices**

570 Recent Advances in Nanofabrication Techniques and Applications

are cut out from the metal/insulator spiral heterostructure. Finally, the two thin slices are attached together face to face so that each stripe crosses in a highly clean environment (Ishibashi et al., 2005; Kaiju et al., 2005; Rahaman et al., 2008). Utilizing this DNB structure, we can expect to realize high density memory devices, the crossing point of which can be scaled down to ultimate feature sizes of a few nanometers thanks to their atomic-scale resolution of the film thickness determined by the rate of metal deposition, ranging from 0.01 to 1 nm/s. This DNB structure also gives a huge potential impact and importance of uniting bottom-up structures with top-down systems (Ishibashi, 2003). One element of the DNB structure is called a quantum cross (QC) device, which consists of two metal thin films (nanoribbons) having the edge-to-edge configuration as shown in Fig. 1 (Ishibashi, 2004; Kondo et al., 2006; Kaiju et al., 2008). In this QC device, the area of the crossed section is determined by the film thickness, in other words 1-20 nm thick films can produce 1×1-20×20 nm2 nanoscale junctions. Since the vacuum evaporation has good spatial resolution of one atomic layer thickness, the junction size of QC devices could ultimately be as small as a few ångströms square (10-2nm2). This method offers a way to overcome the feature size limit of conventional optical lithography. When molecularbased self-assembled monolayers (SAMs), such as rotaxanes (Chen et al., 2003), catenanes (Balzani et al., 2000) and pseudorotaxanes (Pease et al., 2001), are sandwiched between the two thin metal films, QC devices can serve as novel non-volatile memory devices and switching devices. Moreover, when magnetic materials, such as Fe, Co and Ni, are used for the two thin metal films, QC devices can work as nanoscale spin injectors and tunneling magnetoresistance (TMR) devices. Among these devices, the resistance of the electrodes (thin metal films) can be reduced down to ~k since the width of films can be easily controlled to the one as long as ~mm. This makes it possible to realize a highly sensitive detection for a junction resistance and to apply QC devices to high-frequency

In this chapter, we present structural and electrical properties of Ni/polyethylene naphthalate (PEN) films used as electrodes in QC devices (Kaiju et al., 2009 & 2010) and current-voltage (*I-V*) characteristics for three types of QC devices. The three types of QC devices are as follows: (i) Ni/Ni QC devices (17 nm linewidth, 17×17 nm2 junction area), in which two Ni thin films are directly contacted with their edges crossed (Kondo et al., 2009; Kaiju et al., 2010), (ii) Ni/NiO/Ni QC devices (24 nm linewidth, 24×24 nm2 junction area), in which NiO thin insulators are sandwiched between two Ni thin-film edges (Kaiju et al., 2010) and (iii) Ni/poly-3-hexylthiophene (P3HT): 6, 6-phenyl C61 butyric acid methyl ester (PCBM)/Ni QC devices (16 nm linewidth, 16×16 nm2 junction area), in which P3HT:PCBM organic molecules are sandwiched between two Ni thin-film edges (Kaiju et al., 2010; Kondo et al., 2010). In our study, we have successfully fabricated various types of QC devices with nano-linewidth and nano-junctions, which have been obtained without the use of electron-beam or optical lithography. Our method will open up new opportunities for the creation of nanoscale patterns and can also be expected as novel technique beyond conventional lithography. Furthermore, we present the calculation results of the electronic transport in Ni/organic-molecule/Ni QC devices and discussed their possibility for switching devices. According to our calculation, a high switching ratio in excess of 100000:1 can be obtained under weak coupling condition. These results indicate that QC devices fabricated using thin-film edges can be expected to have potential application in next-generation switching devices with

devices.

ultrahigh on-off ratios.

The fabrication method of QC devices is shown in Fig. 2. First, Ni thin films were thermally evaporated on PEN organic substrates (2 mm width, 10 mm length and 25 m thickness) in a high vacuum chamber at a base pressure of ~10-8 torr. PEN organic substrates of TEONEX Q65 were supplied by Teijin DuPont Japan and cut down from 5 to 2 mm width using a slitter in a clean environment. A boron nitride crucible N-1, made by DENKA, and a tungsten filament, made by CRAFT, were used for the thermal evaporation of Ni thin films. A heat-block stainless plate with a hole was inserted between the Ni vapor source and the PEN substrate. The length of the crucible and the aperture size in the stainless plate were designed using a geometrical simulation to evaporate uniform Ni films in-plane to PEN substrates. The temperature near PEN substrates was less than 62 °C, which was lower than the glass transition temperature *T*g of 120 °C for PEN substrates. The pressure during the evaporation was 10-5 torr and the growth rate was 1.0-1.5 nm/min at an evaporation power of 280-350 W.

Then, fabricated Ni/PEN films were sandwiched between two polymethyl methacrylate (PMMA) resins using epoxy. The volume of the PMMA resin was 6 × 3 × 3 mm3. The edge of PMMA/Ni/PEN/PMMA structure was polished by chemical mechanical polishing (CMP) methods using alumina (Al2O3) slurries with particle diameters of 0.1, 0.3 and 1.0 m. The polishing pressure was 6.5 psi and the platen rotation speed was 75 rpm. Finally, two sets of polished PMMA/Ni/PEN/PMMA structures were prepared and attached together with their edges crossed in a highly clean environment of ISO class minus 1. The attachment pressure was 0.54 MPa and no glue was used. This is a basic process in fabricating QC devices. When

Lithography-Free Nanostructure Fabrication Techniques Utilizing Thin-Film Edges 573

oC by liquid nitrogen conduction cooling. The *I-V* characteristics of QC devices were

Figs. 3(a) and (b) show the cross-sectional TEM images for Ni (20 nm)/PEN films. It can be seen that there is no diffusion of Ni into the PEN layer, resulting in clear and smooth formation of the Ni/PEN interface. Here, it should be noted that some researchers have reported that metal atoms diffuse into organic layers in the process of the metal evaporation onto organic layers (Tarlov, 1992; Hirose et al., 1996; Ito et al., 1999; Dürr et al., 2002). For example, the metastable atom electron spectroscopy (MAES) spectra of Au on the p-sexiphenyl (6P)/Au system shows that the features of 6P remain even though Au was deposited to about 20 nm thickness (Ito et al., 1999). This indicates that Au atoms or clusters penetrate into the 6P films. The soft x-ray photoemission spectroscopy (SXPS) investigation of the interface between evaporated indium and perylenetetracarboxylic dianhydride (PTCDA) also demonstrates that the interfacial region is very wide, ranging from 7 to 60 nm, and this means that the metal atoms of indium diffuse into PTCDA organic layers (Hirose et al., 1996). Moreover, according to studies on the interaction between evaporated Ag and octadecanethiol (ODT) on Au films using XPS, Ag deposited at 300 K migrates through the ODT layer and resides at the ODT/Au interface (Tarlov, 1992). As compared with above results, such a metal diffusion into organic layers does not occur in Ni/PEN interface. This indicates that Ni thin films on PEN organic substrates are suitable for metal/organic films used in QC devices. It can also be confirmed that the surface of Ni films is smooth, and this smoothness is in good agreement with the results of the AFM observation, where the surface roughness *R*a is 1.1 nm. Fig. 3(c) shows the electron diffraction (ED) pattern for the same specimen. Ni thin films on PEN films have been shown to be face-centered-cubic structures, which are equal to those in bulk Ni structures. The ED pattern also shows that Ni thin films have polycrystalline structures, which can be recognized from the cross-sectional TEM image of Fig. 3(a). Thus, Ni /PEN films are suitable

for QC devices from the viewpoint of the Ni/PEN interfacial and internal structures.

Fig. 3. (a) Cross-sectional TEM image, (b) high-resolution cross-sectional TEM image and (c)

ED pattern for Ni (20 nm)/PEN films.

measured by a four-probe method at room temperature.

**3. Ni/PEN films used as electrodes in QC devices** 

**3.1 Cross-sectional TEM images of Ni/PEN films** 

Ni/Ni QC devices are fabricated, two Ni thin films are directly contacted with their edges crossed, as shown in Fig. 2(a). For the fabrication of Ni/NiO/Ni QC devices, after two sets of polished PMMA/Ni/PEN/PMMA structures were prepared, the Ni edge in one of the two samples was oxidized by O2 plasma at a power of 5 W (= 5 mA and 1 kV) and then the oxidized and the unoxidized Ni edges were attached together with their edges crossed, as shown in Fig. 2(b). Also, for the fabrication of Ni/P3HT:PCBM/Ni QC devices, after two sets of polished PMMA/Ni/PEN/PMMA structures were prepared, P3HT:PCBM organic molecule blend was sandwiched between two sets of PMMA/Ni/PEN/PMMA structures whose edges were crossed, as shown in Fig. 2(c). P3HT and PCBM organic molecules were separately dissolved in monochlorobenzene, then blended together with a weight ratio of 1:1 to form a 20 mg/ml solution. This P3HT:PCBM solution was dripped on one of the polished PMMA/Ni/PEN/PMMA structure, then sandwiched by the other of the polished PMMA/Ni/PEN/PMMA structure. These three types of QC devices are shown in Fig. 2.

Fig. 2. Fabrication process of (a) Ni/Ni QC devices, (b) Ni/NiO/Ni QC devices and (c) Ni/P3HT:PCBM/Ni QC devices.

#### **2.2 Evaluation methods of Ni/PEN films and QC devices**

The Ni thickness was measured by a mechanical method using the stylus surface profiler DEKTAK and an optical method using the diode pumped solid state (DPSS) green laser at a wavelength of 532 nm and the photodiode detector. The surface morphologies of Ni/PEN samples were analyzed by atomic force microscope (AFM) Nanoscope IIIa. The microstructures as well as the Ni/PEN interfacial structures were examined using a JEOL JEM-3000F transmission electron microscope (TEM) operating at 300 kV. The cross-sectional TEM samples were prepared by a combination of mechanical polishing and Ar ion thinning. To reduce the beam-heating effects during ion thinning, the sample stage was cooled to -160 oC by liquid nitrogen conduction cooling. The *I-V* characteristics of QC devices were measured by a four-probe method at room temperature.
