Nanotube sensor

Description of equivalent WO 2004048958 A2

NANOTUBE SENSOR BACKGROUND The invention pertains to gas sensors. Particularly, it pertains to nanotube sensors. Certain attempts have been made to use nanotube for gas sensing. U.S. Patent Application Serial No. 10/100,440, filed March 18, 2002 and entitled, "Carbon Nanotube Sensor," is hereby incorporated by reference.

SUMMARY Nanotubes can be grown in-situ in a directed manner in a certain gas environment by bridging a moat between two moveable finger-or arm-like support pieces, connections or electrodes for the nanotubes. A manufacturable MEMS-base nanotube sensor can be made. BRIEF DESCRIPTION OF THE DRAWINGS

Figures la and lb show a sensor structure that supports a nanotube.

Figures 2a and 2b show a sensor structure that has a nanotube. Figures 3a and 3b show electrical configurations of a nanotube sensor.

Figure 4 is a graph of current-gate voltage curves of several gases absorbed by a nanotube of a sensor. Figure 5 is a flow diagram of a process of a nanotube sensor. Figures 6a and 6b show a nanotube sensor having a cross-bar like structure.

Figures 7a and 7b are cross-section views of a nanotube sensor similar to that in Figure 6a. Figures 8a and 8b are cross-section views of a nanotube sensor similar to that in Figure 6b.

DESCRIPTION Figure la shows a basic structure that supports a nanotube. Bimorph finger- or arm-like longitudinal structures 11 and 12 are situated on substrate 10. A pedestal-like structure 13 may be situated between the ends of structures 11 and 12 on substrate 10. Structure 13 may be utilized as a heat sink or an electrode or both. Structures 11 and 12 may be composed of a top metal layer 21 and a bottom 22 dielectric, or vice versa. These structures may have other compositions. In figure lb, structures 11 and 12 may be heated and their ends will move away from structure 13 and substrate 10 due to the heat.

In figure 2a, while fingers or arms 11 and 12 are raised up, one or more nanotubes may be grown bridging the two closer ends of structures 11 and 12. This growing may be performed in an atmosphere or local environment of ethylene, methane, CO or the like. A nanotube or nanotubes are grown upon heating the structures to between approximately 700 and 900 degrees Celsius. Generally, one may result in having a carbon single-walled (i.e., one layer of molecules) nanotube 15 bridging the ends facing each other of structure 11 and 12. The nanotube may grow by virtue of flow and/or an electric field from one end to the other of structures 11 and 12. When device 14 is removed from the heat of a furnace or other heat source, structures 11 and 12 cool and their ends 16 and 17 move towards substrate 10 as shown in figure 2b. This activity causes nanotube 15 to sit on heat sink/electrode 13. Structure 13 may be composed of a metal 19 that has been coated with a thin passivation layer 18. Device 14 can be cut out as a die and mounted on a header for operation.

When nanotube 15 has cooled down, it may be exposed to a fluid and absorb some of it. A fluid may be a gas or a liquid. "Gas" is to be referred to in this description but may be interchanged with "liquid". When device 14 is heated as a die, nanotube 14 is lifted off of post or pedestal 13 by structures 11 and 12 when the latter bend as a result of their bimorph structures being heated. When nanotube 15 is heated, absorbed gases are driven off nanotube 15, making it ready for resorption of new gases. When the heat is removed, arms 11 and 12 come down; nanotube 15 sits on heat sink 13 which removes heat from nanotube 15. Then nanotube 15 is ready for resorption of new gas in the sensor' s immediate environment .

To lift nanotube 15 off of heat sink 13, arms 11 and

12 may be actuated several different ways. Device 14 as a die on a header 20 may be heated or structure 11 and 12 may have heating elements in them. In either case, nanotube 15 is removed from contact, with heat sink 13 so that nanotube 15 can more rapidly heat up and desorb any gas on it, as shown in figure 2a. After this, heat is removed from header 20 or heating elements 23 in arms 11 and 12 are disconnected. Structures 11 and 12 move towards substrate 10 and nanotube 15 rests on heat sink

13 to further cool off and be ready for absorption of a new gas.

Figure 3a shows an electrical configuration which may be used to aid in identifying a gas absorbed by nanotube 15. Power supply 24 is connected to structure 11 and 12 which become connections to nanotube 15. The current indication from meter 26 and the voltage indication from meter 25 may be noted, and from such IV characteristics, information about or identification of a gas or liquid absorbed by nanotube 15 may be obtained.

Figure 3b shows a three terminal electrical configuration resembling a field effect transistor having a power supply 27 and a meter 28 for measuring gate- voltage. Structure 13 is the gate and structures 11 and 12 are the source and drain, respectively. The other electrical aspects of this figure are similar to those shown in figure 3a. The current (I) meter 26 and the gate voltage (V_g) meter 28 may be noted, and from such IV_g characteristics, information about or identification of a gas or a liquid absorbed by nanotube 15 may be obtained. On the other hand, for a predetermined V_g set by power supply 27, the current from meter 26 and the voltage from meter 25 may be noted, and from such IV characteristics, information about or identification of a gas or liquid absorbed by nanotube 15 may be obtained. The above readings may be taken periodically over a period of time from the moment that a gas begins to be absorbed by nanotube 15. Such IV and IV_g characteristics may provide additional information, such as concentrations of the gas or liquid in the environment of nanotube 15.

Figure 4 is an example of IV_g characteristics for several gases absorbed by device 14 in figure 3b. Curve 31 shows the IV_g characteristics before gas absorption by nanotube 15. Curve 32 shows that IV_g characteristics after absorption of NH₃ by nanotube 15. Curve 33 shows the IV_g characteristics after absorption of N0₂ by nanotube 15. Absorption of each gas would occur after any gas or liquid in nanotube 15 was desorbed or removed. The nanotubes could be functionalized with different materials (metals, organics, semiconductors) to enhance response and discrimination for different gases. Figure 5 shows a flow diagram 30 outlining the gas/liquid sensing process of device 14 as described above. Block 34 is exposure of nanotube 15 to the gas and/or liquid. Measuring the IV and/or the IV_g characteristics of nanotube 15 is represented by block 35. Next, in block 36, nanotube 15 is moved away from heat sink 13 by support structures 11 and 12 with heat.

Heat is also used to drive off gas/liquid from nanotube 15, as represented by block 37. In block 38, structures 11 and 12 are cooled and return nanotube 15 to rest on heat sink 13 to further cool. Path 39 shows that the sensing process may repeat for sensing another or the same gas or liquid.

Figure 6a reveals a plan view of a sensor 41 having a structure or crossbar 40 that may be a heat sink and/or an electrode of a transistor for purposes of attaining IV_g characteristics of nanotube 15, particularly for gas/liquid sensing. Structures 47 and 48 may be the other connections to the transistor, i.e., nanotube 15. Figures 7a and 7b show a cross-section view of device 41. Nanotube 15 is situated between crossbar 40 and substrate 43. Structures 47 and 48 may have heating elements to heat them and nanotube 15, or substrate 43 may be heated for a similar effect. Upon heating, structures 47 and 48 lower nanotube 15 off of heatsink 40. Upon cooling, nanotube 15 is brought up to heatsink 40 and nanotube 15 is further cooled by structure 40. There may be an inverted pyramid-shaped pit 44 etched in substrate 43, for cooling or other reasons. Some other aspects of device 41 for gas/liquid sensing are like that of device 14.

Figure 6b shows a device 42 which has a crossbar, heatsink or gate-like structure 40 situated between nanotube 15 and substrate 43. Along with structure 40, structures 45 and 46 which are connected to nanotube 15, and with configurations like those of device 14 in figures 3a and 3b, one may get IV and IV_g data of nanotube 15 with or without an absorbed gas or liquid. Figures 8a and 8b show a cross-section of device 42. Finger- or arm-like longitudinal structures 45 and 46 that hold nanotube 15 are like that of structures 47 and 48 of device 41 except that they move away from substrate 43 rather than towards it, when heated. When structures 47 and 48 cool down, they move towards substrate 43 and nanotube 15 may be set on heatsink structure 40 for further cooling. There may be a pyramid-shaped pit 44 in substrate 43 to possibly improve cooling or facilitate other reasons for device 42. Or there may not be a pit 44. Many aspects of device 42 are like those of devices 14 and 41 for gas/liquid sensing.

Longitudinal structures 45, 46, 47 and 48 may be etched, at least in part, from substrate 43, or be formed on substrate 43. Structure 40 may be made in a similar fashion like that of structures 45, 46, 47 and 48. The structures of devices 14, 41 and 42 may be MEMS technology or be compatible with it. The technology of these devices may be silicon based or of another material. Although the invention has been described with respect to at least one specific embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications .