An unexpected discovery by Sony researchers has led to a fast and easy method involving laser beams to separate metallic and semiconducting single wall carbon nanotubes so they may be patterned for use in semiconducting devices and solar cells.
As shown in U.S. Patent Application 20100003809, FIG. 1, a mixture or thin film composed of semiconducting carbon nanotubes and metallic carbon nanotubes (which are separated from one another) is formed on a substrate. The substrate may be that of glass, quartz, or silicon (with or without SiO2 surface coating). It also includes any material that withstands irradiation with intense energy beams (mentioned later). The mixture composed of semiconducting carbon nanotubes 2 and metallic carbon nanotubes may be deposited on the substrate by in-situ CVD or solution process.
Then, as shown in FIG. 2, the semiconducting carbon nanotubes and metallic carbon nanotubes on the substrate are irradiated with energy beams from the irradiation energy source. The irradiation energy source may be a laser source.
Separating metallic carbon nanotubes from semiconducting carbon nanotubes is a major expense, time consuming and difficult. Sony solves the problem by using laser beams to completely destroy either the metallic or the semiconducting carbon nanotubes. The process results in a cheap and easy selective reaction of metallic carbon nanotubes or the selective reaction of semiconducting metallic carbon nanotubes.
Sony Corporation (Tokyo, JP) Houjin Huang reveal a method for destruction of metallic carbon nanotubes. The method includes irradiating a mixture of semiconducting carbon nanotubes and metallic carbon nanotubes with energy beams (such as laser light), thereby selectively destroying metallic carbon nanotubes or semiconducting carbon nanotubes. The energy beams have energy components for resonance absorption by the metallic carbon nanotubes or semiconducting carbon nanotubes, according to U.S. Patent Application 20100003809.
As the result of their research into separation problems involved in the carbon nanotube production, the Sony inventors "unexpectedly found that metallic carbon nanotubes can be selectively destroyed by irradiation with laser beams."
Presumably, this phenomenon is due to resonance absorption of incident laser beams by metallic carbon nanotubes and destruction of metallic carbon nanotubes by absorbed energy. This means that metallic carbon nanotubes can be selectively destroyed by irradiation with laser beams having energy suitable for resonance absorption by metallic carbon nanotubes to be destroyed.
Likewise, semiconducting carbon nanotubes can also be selectively destroyed by irradiation with laser beams having energy suitable for resonance absorption by semiconducting carbon nanotubes to be destroyed. The mechanism of destruction apparently suggests that the laser beams may be replaced by any other energy beams such as electron beams so long as they have energy components for resonance absorption by metallic carbon nanotubes or semiconducting carbon nanotubes to be destroyed.
The present inventors also found that metallic carbon nanotubes efficiently undergo selective reactions when excited by irradiation with energy beams (such as laser beams) in an environment containing a reactive substance. Similarly, semiconducting carbon nanotubes efficiently undergo selective reactions when excited by irradiation with energy beams (such as laser beams) in an environment containing a reactive substance.
The present inventors also found that metallic carbon nanotubes efficiently undergo selective reactions when excited by irradiation with energy beams (such as laser beams) in an environment containing a reactive substance. Similarly, semiconducting carbon nanotubes efficiently undergo selective reactions when excited by irradiation with energy beams (such as laser beams) in an environment containing a reactive substance.
Attempts to apply single-wall carbon nanotubes to semiconductor electronics on a widespread scale have been unsuccessful so far because they contain both metallic ones and semiconducting ones so long as they are synthesized by any existing technology.
Single-wall carbon nanotubes can be metallic or semiconducting depending on their chirality, which is an angle at which the graphite lattice (or graphene sheet) helically rounds about the tubular contour of the nanotube. Metallic carbon nanotubes (which account for about one-third of total nanotubes) greatly aggravate the FET characteristics, such as on/off ratio.
It is impossible to adjust the on/off ratio with a network film of untreated carbon nanotubes. In fact, a network film of carbon nanotubes has an on/off ratio lower than 10, which is too small for any practical application. How to address the problem with metallic carbon nanotubes has been a major point in this field.
Sony's method generally relates to a method for destruction of metallic carbon nanotubes, a method for production of aggregate of semiconducting carbon nanotubes, a method for production of thin film of semiconducting carbon nanotubes, a method for destruction of semiconducting carbon nanotubes, a method for production of aggregate of metallic carbon nanotubes.
It also covers a method for production of thin film of metallic carbon nanotubes, a method for production of electronic device, a method for production of aggregate of carbon nanotubes, a method for selective reaction of metallic carbon nanotubes, and a method for selective reaction of semiconducting carbon nanotubes which can be applied to thin-film transistors (TFT) in which thin film of carbon nanotubes is used as the channel material.
The metallic carbon nanotubes used are preferably single-wall metallic carbon nanotubes; however, it may also include double-wall or multi-wall metallic carbon nanotubes. The metallic carbon nanotubes should have a diameter of 0.4 to 10 nm, preferably 0.4 to 3 nm. The metallic carbon nanotubes should have a length of 1 to 106 nm, preferably 10 to 104 nm. The metallic carbon nanotubes should preferably be in the form of thin film with an average thickness of 0.001 to 105 nm, although they may also be in the form of single substance.
The electronic element is not specifically restricted so long as it has aggregate of metallic carbon nanotubes. It includes, for example, solar cells, photoelectric converters, light-emitting elements, FETs (such as TFTs), memories, and chemical sensors.
Among the promising semiconductor electron materials of next generation is semiconducting single-wall carbon nanotubes. This is because semiconducting single-wall carbon nanotubes exhibit not only better electrical properties than silicon (as the major channel material of TFT) but also outstanding mechanical properties that will permit their application to flexible electronics. A field effect transistor (FET) that operates at room temperature with one single-wall carbon nanotube was realized in 1998 for the first time.
Irradiation in this manner destroys the metallic carbon nanotubes, while leaving the semiconducting carbon nanotubes intact, in the desired regions, thereby creating the regions of semiconducting carbon nanotubes. In other words, it is possible to form the regions of semiconducting carbon nanotubes in any shape at any position. The regions left without irradiation with the energy beams become metallic regions containing the metallic carbon nanotubes remaining intact, and hence these regions may function as electrodes.