Department of Physics

UusiHeaderi

Molecular Technology

The Molecular Technology group studies primarily the experimental electronic and mechanical properties of carbon nanotubes (CNTs) and devices that are based on them. The interests include both fundamental and applied aspects of CNT science and technology. The research in the group has extensively explored the basic electronic transport properties of high quality multiwalled carbon nanotubes (MWNT). Other topics within the group include the interaction between CNTs and liquid interfaces and the functionalization of CNTs with molecular species. The group utilizes for its research effort the modern microscopy instrumentation and the good fabrication and measurement facilities of the Nanoscience Center.

Contact person: Markus Ahlskog

  • Group members »
    • Markus Ahlskog, professor
    • Matti Hokkanen, doctoral student
    • Dongkai Shao, doctoral student
    • Antti Lukkarinen, MSc student
    • Saara Lautala, MSc student
    • Joonas Saari, MSc student
  • Recent research
    • Electronic transport in MWNTs »

      Electronic transport in MWNTs
      a) AFM image of typical MWNT device on Si/SiO2. The highly doped Si substrate acts as a back-gate. b) Gate voltage (VG) dependent conduction at different temperatures of a semiconducting MWNT. c) The bandgap energy vs. diameter D, estimated from experimental values from the low temperature conduction experiments. The black dotted line shows the standard tight-binding theory value (Eq. 1) for the diameter dependent bandgap EG of semiconducting SWNTs.

      The published works on the fundamental science of single wall carbon nanotubes (SWNT) is many times more numerous than that on multiwalled carbon nanotubes (MWNT). One major reason for this is that SWNTs appear in both metallic and semiconducting forms, while in MWNTs mainly diffusive/quasiballistic metallic states have previously been reported, in a few experimental works on MWNTs electronic low temperature transport and high magnetic field properties. However, these studies have been fragmentary. For example, before our work [1,2], there was no consistent, experimentally verified description of semiconductivity in MWNTs. Transport properties on MWNTs have to date not been probed systematically at different diameters. In particular, for reasons mainly due to synthesis technology, there has been very few reports on smaller MWNTs with the diameter within the interesting range 3 - 10 nm. This has left the experimental studies on MWNTs rather disconnected from those on SWNTs and DWNTs.

      In most studies on MWNTs, the working assumption has been that the outer layer, or possibly few layers, is solely responsible for the low bias transport properties. One motivation for this assumption is the very large anisotropy of conductance in graphite and few layer graphene. In principle, one should find among MWNT-based devices a division into metallic or semiconducting types, for example with respect to the outer layer. In semiconducting SWNTs, the conventional tight-binding theory calculation, gives for the dependence of the bandgap (EG) on diameter (D) as:

                               EG = b / D                                          [1]

      where b ≈ 0.7 eVnm. Thus, in a first approximation, Eq. 1 is expected to apply to the semiconducting outer layers of MWNTs. The semiconducting properties of a MWNT can be measured, at least qualitatively, in a three-terminal field-effect device configuration, (a) in Figure above, where its bandgap shows up as a transport gap, which is the range of gate voltages where the conductance decreases strongly or vanishes, as shown in (b) above.

      Low temperature transport in MWNTs has been studied at different diameters and lengths, within 2 - 10 nm, and 0.3 - 3.5 mm, respectively [2]. In a majority of the samples, semiconductivity showed up as a transport gap in the gate voltage controlled conduction, but metallic MWNTs are found in all diameters. The transport gap is seen to be quantitatively determined by a diameter dependent bandgap, and length dependent localization of charge carriers. From an analysis of about 80 devices, we obtain an estimate for the bandgap of semiconducting MWNTs, as shown in (c) above. This bandgap is estimated to be smaller than that extrapolated from the conventional expression, eq. 1, applicable to semiconducting single wall carbon nanotubes.  

      These results have significant similarities to graphene nanoribbons (GNR), where a gap arises via quantum confinement due to the narrow width. The size of the gap is then roughly in a similar inverse relation with the width, as in the case of the diameter dependence of the MWNT's in our work.

      [1] M. Ahlskog, O. Herranen, A. Johansson, J. Leppäniemi, and D. Mtsuko.  Electronic transport in intermediate sized carbon nanotubes. Physical Review B, 79, (2009) 155408.
      [2] D. Mtsuko, A. Koshio, M. Yudasaka, S. Iijima, and M. Ahlskog, Measurements of the Transport Gap in Semiconducting Multiwalled Carbon Nanotubes with Varying Diameter and Length. Physical Review B. 91, (2015) 195426.




    • On-chip purification of arc-discharge synthesized MWNTs »

      A major difficulty for experimental research on individual MWNT’s is the issue of sample quality. Arc-discharge synthesized MWNT’s (AD-MWNT) are typically of high quality but the macroscopic material contains excessively amorphous carbon. AD-MWNT material has been purified with different methods, but all of these have serious problems in that the MWNT quality suffer from the purification steps. We have made progress [1] in MWNT purification with a method that begins with dispersing the raw MWNT material on a flat surface, e.g. piece of silicon wafer. Then a water surface interface is moved across, whereby a large part of the MWNT deposition is removed by the water surface. This removed part contains some MWNTs and nearly all the impurities. The other part consists of the highly purified and practically intact MWNT’s (Figure 2).

      Purification of nanotubes
      AFM images (10 µm in size) show same location before and after purification treatment.



      [1] Matti J. Hokkanen, Saara Lautala, Shao Dongkai, Tuomas Turpeinen, Juha Koivistoinen, Markus Ahlskog, On-chip purification via liquid immersion of arc-discharge synthesized multiwalled carbon nanotubes, Applied Physics A, 122, (2016) 634.


    • MWNTs and protein adsorption »

      Carbon nanotubes are considered as the active elements in chemical and biochemical sensor devices. For this purpose, the effects of protein adsorption on the conductance have been measured in single CNT devices [1]. In this context, nearly always the CNT is a semiconducting SWNT forming the active channel in a field effect transistor (SWNT-FET). In this work, CNTs have been for the first time employed for electronic detection of hydrophobin (HFBI) protein molecules. HFBI is a surface active protein having both hydrophobic and hydrophilic functional groups which has previously been used for CNT functionalization and solubilization. Our result indicates a decrease in device conductance after exposure to ~ 100 nM NCysHFBI in phosphate buffer solution. This decrease could be drastic when measured in situ in solution.

      Protein adsorption
      a) A schematic picture of the measurement setup, with a SWNT-FET device having a flow cell on top while HFBI protein molecules are injected. b) Time dependence measurement with constant VG, showing the change as HFBI is mixed into the buffer solution.



      [1]  Peerapong Yotprayoonsak, Géza R. Szilvay, Päivi Laaksonen, Markus B. Linder, and Markus Ahlskog The effect of Hydrophobin protein on conductive properties of Carbon Nanotube Field-Effect Transistors; First study on sensing mechanisms.   Journal of Nanoscience and Nanotechnology (JNN), 15, (2015) 2079.

  • Recent publications »

    M. Ahlskog, O. Herranen, A. Johansson, J. Leppäniemi, and D. Mtsuko.  Electronic transport in intermediate sized carbon nanotubes. Physical Review B, 79, (2009) 155408.

    D. Mtsuko, A. Koshio, M. Yudasaka, S. Iijima, and M. Ahlskog, Measurements of the Transport Gap in Semiconducting Multiwalled Carbon Nanotubes with Varying Diameter and Length. Physical Review B. 91, (2015) 195426.

    Matti J. Hokkanen, Saara Lautala, Shao Dongkai, Tuomas Turpeinen, Juha Koivistoinen, Markus Ahlskog, On-chip purification via liquid immersion of arc-discharge synthesized multiwalled carbon nanotubes, Applied Physics A, 122, (2016) 634.