str1-21

Main research areas:

1 Synthesis of nanomaterials2 Transparent, flexible, and stretchable electronics3 Photovoltaics
For decades nanostructures and nanostructured materials attracted a wide attention of researchers in many scientific fields. The main reason for this interest is explained by the transformation of material properties with the size. At the nanorange optical, electrical, thermal and mechanical properties of the materials are defined by their extremely high specific surface area. The properties of the nanomaterials significantly differ from those of bulk materials and individual atoms/molecules comprising the structure.

LNM group is working on the synthesis of single-walled carbon nanotubes, graphene-based materials and metal-based nanowires.

Single-walled carbon nanotubes (SWCNTs) are a unique material with diverse exceptional properties, which could be utilized in many fields of science and technology. SWCNTs are the strongest known material with exceptionally high Young's modulus of elasticity and tensile strength. SWCNTs possess very high porosity and high specific surface area. The most fascinating properties of SWCNTs are their electronic structure, which provides many interesting applications in photonics and electronics. SWCNT's optical and electrical optical and electrical properties can be varied by changing the chirality from semiconducting to metallic SWCNTs (Fig. 1). Both thermal and electrical conductivities of SWCNTs show remarkably high values. The charge carrier mobility in the semiconducting SWCNTs is extremely high and comparable to that of free-standing graphene. Due to very high current density, which tubes can withstand without destruction, up to 109 A/cm2, SWCNTs are believed to be an ideal material for copper and aluminium replacement in integrated circuits. Taking into account high transparency, SWCNT films are a strong candidate for the replacement of commonly used transparent electrodes, such as ITO. The group developed synthesis of SWCNTs by aerosol1-5 and substrate6-10 CVD synthesis. Both methods allow synthesising high quality SWCNTs (e.g., Fig. 2).

Graphene possesses very similar properties to SWCNTs. Graphene is strictly two-dimensional material exhibits exceptionally high crystal and electronic quality and, despite its short history, has already revealed a cornucopia of new physics and potential applications. It is tougher than diamond, but stretches like rubber. It is very transparent and conducts electricity and heat better than copper nanowires. The importance of graphene as a material for flexible optoelectronic applications is not in doubt.

The group is working on the CVD synthesis of graphene and fabrication of graphene layers by deposition and subsequent reduction of graphene oxide. One of the directions of the group is to hybridize the SWCNT and graphene into a uniform single hybrid structure (Fig. 3), which take advantage of the synergistic effects between the tubes and graphene.

LNM group focuses on the aerosol CVD synthesis of SWCNTs and thin film applications of the tubes (Fig. 2). The most important publications related to the synthesis of SWCNTs and investigations of the mechanisms of their synthesis are listed below 1, 3-5, 11-18.

Metal-based nanowires (NWs) can be used for a wide range of applications and especially for nano- and microelectronics19, 20. We have introduced a very simple and rapid method for the synthesis of NWs by resistive heating of metal wires at ambient conditions. We demonstrated the possibility of fabrication of different metal oxide NWs: CuO, a-Fe2O3, ZnO, V2O5 (Fig. 4) and showed the possibility of utilization of other metals – Al, Mo and W for the NW growth. The simplicity of our discovered production method allows elaborating methods for the patterned growth, transferring method as well as high yield synthesis of NWs.

Additionally, we have recently elaborated two methods for the controlled synthesis of ZnO tetrapod (ZnO-T) structures (Fig. 5), which possess very interesting electrical and optical properties20, 21. We demonstrated high performance UV light sensor based on ZnO-T film (Fig. 6). The fabricated flexible transparent UV sensors showed 45 fold current increase under UV irradiation with the intensity of 30 µW/cm2 at the wavelength of 365 nm and response time of 0.9 s, which the shortest response for the sensor made of any other ZnO structures.

Fig. 1. a) Sinlge-walled carbon nanotubes of different charalities: semiconducting zig-zag (10,0) and chiral (8,4) tubes, metallic armchair (6,6) tube. b) A schematic presentation of construction of a zig-zag (10, 0) CNT by rolling a cylinder joining hexagons (0,0) and (10,0). Also, the possibility of creating arm-chair (6, 6) and chiral (8,4) tubes are shown.

Fig. 1

Fig. 1

Fig. 2. Transmission (TEM) and scanning (SEM) electron microscopy images of aerosol synthesised SWCNTs.

Fig. 2

Fig. 2

Fig. 3. Fabrication of a hybrid structure SWCNT and reduced graphene oxide.

Fig. 3

Fig. 3

Fig. 4. SEM images of metal oxide NWs: -Fe2O3, CuO, V2O5, ZnO.

Fig. 4

Fig. 4

Fig. 5. a) ZnO-Ts synthesized at 700 oC by aerosol method. b) ZnO-Ts covered by 15 nm-sized Au particles.

Fig. 5

Fig. 5

Fig. 6. ZnO-T UV sensor: a) schematics of UV response measurement inset is a photo of the sensor; b) the sensor response to excitation at 365 nm.

Fig. 6

Fig. 6
Applications of the SWCNT films depend on their thicknesses. Thin SWCNT networks with density close to the percolation threshold exhibit semiconducting behaviour suitable as an active layer for the fabrication of thin-film transistors and sensors. Thicker films (up to 100 nm) possess high transmittance and low resistivity and can be utilized for fabrication of transparent and flexible electrodes (ITO replacement, Fig. 7). High porosity thick films are excellent materials for fuel and solar cells, supercapacitor, battery applications and also gas filtering. The transparent, flexible, stretchable and conductive films can be used in a wide range of applications such as displays (LCD, plasma and touch screens, e-paper, etc.), lighting (light-emitting electrochemical cells, LEECs, and organic light emitting diodes, OLEDs) and photovoltaic (PV) devices.

One cannot even imagine how our world would change, if materials with high conductivity, transparency and flexibility were available in the market. New generation electronic devices will be invented and developed. Novel concepts from World leading electronic companies (Apple, Samsung, HuaWei, Lenovo, LG, Nokia, HTC, etc.) predict the revolutionary changes in the current functionality, shape and view of the electronic devices (Fig. 8).

We elaborated a method for the room temperature deposition of CNTs onto any substrate22, 23, which presents an interesting and useful nanotechnological applications (Fig. 9, MOVIE 1). We proposed a method to directly integrate the CNTs into applications without time-consuming sample purification, dispersion them in liquid and subsequent deposition. Since the CNTs are clean, they can be directly utilized in the form they come from the reactor. Since the aerosol CNTs are collected at room temperature, they can be deposited onto any substrate including temperature sensitive polymers (Fig. 10). CNTs simply filtered and subsequently transferred onto the secondary support demonstrate a state-of-the-art performance of thin and transparent electrodes with the performance approaching ITO (Fig. 11) 4, 5,22-24.

We have also developed a fabrication method of high-performance thin film field effect transistors (FETs) on the basis of deposited SWCNTs from the aerosol reactor. First devices exhibited on/off ratios of up to 105 and charge mobilities of up to 4 cm2V-1s-1.25 Later, in collaboration with Prof. Y. Ohno we reported significant increase of the transistor mobility up to 634 cm2V-1s-1 at the on/off ratio of 6x106. Also, we demonstrated the possibility to build flexible integrated circuits, including a 21-stage ring oscillator and master-slave delay flip-flops that are capable of sequential logic (Fig. 12)26, 27.

Importantly, the SWCNTs synthesized by the aerosol method [25,113,163] meet all the requirements for low-cost, flexible, transparent and stretchable electronics, which could be used in many high tech applications like touch sensors and displays, thin film transistors, flexible and transparent electrodes in photovoltaic devices, field emitters, optoelectronic devices, OLEDs, etc.14, 22-24, 26-32

Fig. 7. Applications of transparent conductors.

Fig. 7

Fig. 7

Fig. 8. The concept of flexible and stretchable electronic devices.

Fig. 8

Fig. 8

Fig. 9. Dry and direct deposition method of SWCNT films.

Fig. 9

Fig. 9

Fig. 10. a) A photograph of a filter covered by SWCNTs after filtering the flow at the outlet of the reactor and various substrates with dry transferred SWCNT-networks: on PET, silicon wafer, Fe and Cu foils. b) Multifunctional free-standing SWCNT film.

Fig. 10

Fig. 10

Fig. 11. Comparison of different transparent electrode materials: sheet resistance versus optical transmittance at 550 nm. Our current data are shown by open and solid pentagons; the red lines show theoretical fits to our data.

Fig. 11

Fig. 11

Fig. 12. CNT TFTs and ICs on flexible substrate. a) Photograph of fabricated devices on a flexible and transparent PEN substrate. b) Schematic cross section of a bottom gate TFT on PEN substrate with Al2O3 gate insulator. c) Transfer characteristics of a typical TFT with =100 Ојm at =-0.5 V, =100 Ојm. d) Input-output and gain characteristics of an inverter. Insets show the optical micrograph, circuit diagram and symbol of the inverter. e, f) Optical micrograph and circuit diagram of a 21-stage ring oscillator. g) Output characteristics of the ring oscillator with oscillation frequency of 2.0 kHz at =-4 V. h) Mobility and on/off ratio. Comparison between our 36 CNT TFTs with other representative TFTs based on CNT network, amorphous-Si, polycrystalline Si, ZnO-based semiconductors, and organic materials.

Fig. 12

Fig. 12
Our group is specialised on solar energy applications of the produced materials: SWCNTs, graphene, and nanowires. Solar energy is the most abundant and yet least harvested sources of renewable energy. In recent years, tremendous progress has been made to develop next generation photovoltaic devices based on nanomaterials that have potential for mass production. In this view of cost-effective solar cells, several novel device structures and materials processing techniques have been explored that can enable acceptable efficiencies. Nowadays, the advances in nanotechnology have well understood several properties of nanomaterials that have made them potential candidates for solar cell applications. There are many advantages in using nanostructure-based solar cell devices such as reduced manufacturing cost, unlimited material availability and good quantum efficiency achievable by exploiting their low-dimensional optoelectronic properties that permit multiple electron-hole pair generation.

SWCNTs possess a wide range of direct bandgaps matching the solar spectrum, strong photoabsorption, from infrared to ultraviolet, and high carrier mobility and reduced carrier transport scattering, which make themselves ideal photovoltaic material.

Research on a new kind of heterojunction solar cells is based on silicon and SWCNT films33 (Fig. 13). Detailed insight of the heterojunction of silicon and SWCNTs and the charge transfer mechanism in the CNT network is necessary. This will open up the path towards to all carbon solar cells (Fig. 14)28. We are interested in a deep understanding of the processes limiting the life time and collection efficiency of charge carriers as well as photon absorption in this device structure. Our work focuses on flexible heterojunction solar cells, which will be based on nanocrystalline and amorphous silicon to further improve the mechanical properties of the solar cells (to add flexibility and stretchability).

Fig. 13 (a) Schematic of the SWNT/Si solar cell with dimensional parameters. (b) Magnified SEM image of contact window edge of the SWNT/Si solar cell. A region of the SWNT film with the width of 300 nm was suspended between the Si substrate and the Pt electrode. d) J-V characteristics of the SWNT/Si solar cell measured immediately as well as after 1, 6 and 10 months exposed in the ambient condition.

Fig. 13

Fig. 13

Fig.14. Demonstration of all-carbon nanotube photovoltaic effect: a) The edge of the SWNT film observed under SEM, and b) SEM image of this film showing SWNT network. c) Schematic view of the contact formation between two SWNT films: Yellow stars symbolize the junctions between bundles, (1)–(3) represent three different inclination angles. d) I–V characteristics at “first touch” conditions: under dark and light illumination.

Fig. 14

Fig. 14

 

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