Query: Understanding Thickness-Dependent Electrical Characteristics in Conjugated Polymer Transistors With Top-Gate Staggered Structure

Candidate papers:

abstract: Ketopyrrolopyrrole thieno[3,2 b] thiophene (DPPT TT) and indacenodithiophene co enzothiadiazole (IDT BT) were employed for organic field effect transistors (OFETs) to understand the effects of semiconductor channel thickness on electrical properties. The mobility was found nearly constant, whereas the threshold voltage, contact resistance, and subthreshold slope reached an optimum at a semiconductor layer thickness of about 40 nm. This value was related to the height of the source/drain electrodes. The device performance could be degraded when the thickness of semiconductor film is higher or lower than this critical value. Two different mechanisms are proposed to explain the experimental results.
author_names: Fanming Huang

Mengjiao Li

Yangtao Xu

Anyang Cui

Wenwu Li

Yong Xu

Junhao Chu

Yong-Young Noh
title: Understanding Thickness Dependent Electrical Characteristics in Conjugated Polymer Transistors With Top Gate Staggered Structure
venue: IEEE Transactions on Electron Devices
year: 2019

abstract: Introduction Solution processable n channel organic field effect transistors (OFETs) with high electron mobility have attracted growing interest because they enable low cost production of flexible CMOS like circuits based on OFETs. It is well known that electron transport in OFETs is strongly influenced by the chemical species of substrates such as hydroxyl groups and film crystallinity, and the hydrophobic treatment of substrates is essential to achieve high performance in bottom gate n channel OFETs. We have reported that the use of a top gate configuration enables extraction of high field effect mobility and better electrical stability in OFETs based on solution processable p type organic semiconductors without using hydrophobic substrates [1 3] Here, we report the performance of solution processed top gate n channel OFETs using a novel naphthalene diimide based n type conjugated polymer semiconductor, end capped poly{[N,N' bis(2 decyl tetradecyl) naphthalene 1,4,5,8 bis(dicarboximide) 2,6 diyl] alt 5,5' (2,2' biselenophene)(ePNDIBS) [4] Experimental The structure of n channel top gate OFETs fabricated in this study is schematically shown in Fig. 1. An electron injection layer of Cs2CO3 was formed on Au electrodes by spin coating from 2 ethoxyethanol. The ePNDIBS thin films were fabricated by spin coating chloroform based solutions, followed by annealing at 200 C. A fluoropolymer insulator CYTOPTM (CTL 809M, Asahi Glass) was spin coated onto ePNDIBS films and Al gate electrodes were evaporated on CYTOP layers to complete OFET fabrication. The preparation of organic films and FET measurements were performed in a N2 filled glove box. Results and discussion Typical output and transfer characteristics of top gate ePNDIBS FETs are shown in Fig. 2, together with the variation in transfer curves before and after a gate bias stress for 10 4 s. The fabricated top gate OFET devices exhibit a very stable operation with a negligible shift of threshold voltage and show a high electron field effect mobilities (average: 0.30 cm 2 V 1 s 1 highest: 0.51 cm 2 V 1 s 1 These values are comparable to or higher than the mobility reported in bottom gate devices with octyltrichlosilane treated SiO2 insulators (0.24 cm 2 V 1 s 1 [4] The results indicate that a highly ordered microstructure is spontaneously formed at the surface of solution processed ePNDIBS thin films similarity to regioregular polythiophenes [2,3] and the use of the top gate configuration is also effective for improving solution processable n channel OFETs. References [1] T. Endo et al. Appl. Phys. Express 3, 121601 (2010) [2] T. Kushida et al. Appl. Phys. Lett. 98, 063304 (2011) [3] K. Takagi et al. Org. Electron. 15, 372 (2014) [4] Y. J. Hwang, N. M. Murari, and S. A. Jenekhe, Polym. Chem. 4, 3187 (2013) Acknowledgements This work was financially supported by a Grant in Aid for Scientific Research on Innovative Areas "New Polymeric Materials Based on Element Blocks" (No. 24102011) from the MEXT and a Grant in Aid for Scientific Research (C) (No. 26420276) from the JSPS, Japan.
author_names: Yu Suenaga
title: High Performance Top Gate Organic Field Effect Transistors Based on n Type Polymer Semiconductors
year: 2015

abstract: Abstract Semiconducting single walled carbon nanotubes (semi SWNTs) are attractive as they can be used enable high performance nano electronic devices. We report the enhanced ambipolar charge transport characteristics of the semi SWNT field effect transistors (FETs) based on a top gated staggered device structure. SWNTs were selectively dispersed in non polar organic solvents and sorted by a p conjugated polymer wrapping method. The sorted semi SWNTs solution was used simply to form a well percolated CNT network, and the top gate and bottom contact FETs showed relatively high and equivalent electron and hole mobilities with very high on/off current ratios and steep subthreshold slopes. The equivalent ambipolar charge transport behavior of semi SWNTs was used to demonstrate a reliable complementary like electronic circuits. The inverters showed a good switching threshold near the ideal point at half the driving bias, high gain, low hysteresis, and stability under repeatable operating conditions. They can thus be broadly applied as a fundamental circuit element in printed and flexible electronics.
author_names: Kang-Jun Baeg

Hee Jin Jeong

Seung Yol Jeong

Joong Tark Han

Geon-Woong Lee
title: Enhanced ambipolar charge transport in staggered carbon nanotube field effect transistors for printed complementary like circuits
year: 2017

abstract: Since the discovery of conductivity in polymers and certain organic molecules, remarkable progress has been made in synthesizing organic materials, in understanding their properties and in developing them for use in electronic and optical devices as stated in A. J. Heeger (2001) Currently, polymer and organic light emitting diodes (LEDs) based in R. H. Friend et al. (1999) photovoltaic cells based in C. J. Brabec et a. (2003) and field effect transistor based in N. Stutzmann et al. (2003) are being pushed towards commercialization. Among all the conjugated polymers, poly(l,4phenylenevinyiene) or PPV as well as its derivative 2 methoxy 5 (2 ethylhexyloxy) p phenylenevinylene or MEH PPV have been extensively studied for LED applications. We present here a study of a single layer structure LED with MEH PPV as the active organic layer. Optical and electrical characterization completes the study of the electroluminescence obtained from such device.
author_names: Stephanie Cheylan

Goncal Badenes

Joaquim Puigdollers

Cristobal Voz

Ramon Alcubilla
title: Optical and electrical characteristics of LEDs based on a single organic layer
venue: Conference on Electron Devices, 2005 Spanish
year: 2005

abstract: TESI DI DOTTORATO A Study on Defects in Organic Semiconductors for Field Effect Transistors CANDIDATO: ROSALBA LIGUORI TUTOR: PROF. ALFREDO RUBINO COORDINATORE: PROF. ANGELO MARCELLI Anno Accademico 2012 2013 The understanding and the modeling of mechanisms involved in organic semiconductors are the aims of this Ph.D. thesis. In particular, the document focuses the attention on the role played by organic semiconductor defects on the electrical performance of organic based field effect transistors. Critical issues are, indeed, the localized states related to the presence of structural defects and chemical impurities. They dominate the charge carrier transport in organic semiconductors and define the quality of interfaces occurring in the transistors. Organic thin film transistors were fabricated with pentacene, a conjugated small molecule exhibiting p type behavior, in combination with various polymeric gate insulators. The relationship between the electrical characteristics and the physical processes of the organic devices was investigated, along with a study of their electrical stability, essential to allow the adoption of organic transistors in practical applications. Most of the unstable behaviors observed in the fabricated devices were justified by assuming a multiple trapping and release mechanism to occur, due to the high concentration of traps. Particular analyses of the physical properties of organic materials were presented, exploiting in detail the photo excitation of charge carriers to produce hidden phenomena, which were used for the characterization of localized states. Thanks to this expedient, interesting results were obtained and, in particular, a novel analytical model describing the creation and annihilation dynamics of metastable defects induced in a pentacene film by an ultraviolet irradiation was developed. The evolutions of two types of induced defects, referred to as slow and fast defects, were reconstructed through the proposed equations. Induced defects were demonstrated to control the degradation and recovery kinetics of conductivity. The physical mechanisms occurring in organic transistors at the semiconductor insulator interface, where the conducting channel is formed, were studied through photocurrent and photocapacitance measurements and were revealed to dominate important aspects of device performance and stability under realistic atmospheric conditions. For this reason, specific equations were developed to model the combined effects induced by light irradiation and bias stress. The proposed model allowed a simultaneous study of the light effect on material structure, the trapping process of minority charge carriers at the insulator semiconductor interface and the photoconductive efficiency in the organic semiconductor. Moreover, the new parameters introduced to describe the photoresponse evolution become characteristic figures of merit, fundamental to compare different organic devices by including information about the illumination time and the bias during measurements. Finally, a significant contribution to the modeling of an organic thin film transistor was given through the development of a complete electrical model able to describe the dynamics of the various processes occurring in the organic device. By exploiting the potentialities of admittance spectroscopy in an organic based metal insulator semiconductor capacitor, an equivalent circuit was built and increasingly improved though the introduction of new components. Thus, every element was connected to each other through different relationships, each describing a single process in the device, including the diffusion of mobile ions, the dispersive transport in pentacene bulk, the trapping mechanism at the insulator semiconductor interface and the contact resistance at the metal organic interface. The simulation results showed that the proposed analytical equations are very effective in predicting the electrical behavior of organic devices. This model, suitable for device simulations, also provides an efficient parameter extraction method, useful to compare the device properties with respect to the geometries, the materials and the deposition conditions.
author_names: Angelo Marcelli
title: A Study on Defects in Organic Semiconductors for Field Effect Transistors
year: 2014

abstract: Organic semiconductors have unique properties compared to traditional inorganic materials such as amorphous or crystalline silicon. Some important advantages include their adaptability to low temperature processing on flexible substrates, low cost, amenability to high speed fabrication, and tunable electronic properties. These features are essential for a variety of next generation electronic products, including low power flexible displays, inexpensive radio frequency identification (RFID) tags, and printable sensors, among many other applications. Accordingly, the preparation of new materials based on p conjugated organic molecules or polymers has been a central scientific and technological research focus over the past decade. Currently, p channel (hole transporting) materials are the leading class of organic semiconductors. In contrast, high performance n channel (electron transporting) semiconductors are relatively rare, but they are of great significance for the development of plastic electronic devices such as organic field effect transistors (OFETs) In this Account, we highlight the advances our team has made toward realizing moderately and highly electron deficient n channel oligomers and polymers based on oligothiophene, arylenediimide, and (bis)indenofluorene skeletons. We have synthesized and characterized a "library" of structurally related semiconductors, and we have investigated detailed structure property relationships through optical, electrochemical, thermal, microstructural (both single crystal and thin film) and electrical measurements. Our results reveal highly informative correlations between structural parameters at various length scales and charge transport properties. We first discuss oligothiophenes functionalized with perfluoroalkyl and perfluoroarene substituents, which represent the initial examples of high performance n channel semiconductors developed in this project. The OFET characteristics of these compounds are presented with an emphasis on structure property relationships. We then examine the synthesis and properties of carbonyl functionalized oligomers, which constitute second generation n channel oligothiophenes, in both vacuum and solution processed FETs. These materials have high carrier mobilities and good air stability. In parallel, exceptionally electron deficient cyano functionalized arylenediimide derivatives are discussed as early examples of thermodynamically air stable, high performance n channel semiconductors; they exhibit record electron mobilities of up to 0.64 cm(2)/V*s. Furthermore, we provide an overview of highly soluble ladder type macromolecular semiconductors as OFET components, which combine ambient stability with solution processibility. A high electron mobility of 0.16 cm(2)/V*s is obtained under ambient conditions for solution processed films. Finally, examples of polymeric n channel semiconductors with electron mobilities as high as 0.85 cm(2)/V*s are discussed; these constitute an important advance toward fully printed polymeric electronic circuitry. Density functional theory (DFT) computations reveal important trends in molecular physicochemical and semiconducting properties, which, when combined with experimental data, shed new light on molecular charge transport characteristics. Our data provide the basis for a fundamental understanding of charge transport in high performance n channel organic semiconductors. Moreover, our results provide a road map for developing functional, complementary organic circuitry, which requires combining p and n channel transistors.
author_names: Hakan Usta

Antonio F Facchetti

Tobin J Marks
title: n Channel semiconductor materials design for organic complementary circuits.
venue: Accounts of chemical research
year: 2011

abstract: Author(s) Phan, Hung Advisor(s) Nguyen, Thuc Quyen Abstract: The Field Effect Transistor (FET) is inarguably the most important circuit element in modern electronics. Metal Oxide FETs (MOSFETs) the most common type of FET, are integrated in microprocessors in almost all electronic devices: mobile phones, computers, digital cameras, and digital printers, to name a few. MOSFETs are normally fabricated on top of rigid single crystalline silicon, which requires high temperature processing ~1000 oC) Recently, a need has emerged for flexible electronics in a variety of applications. Examples include form fitting healthcare monitoring devices, flexible displays, and flexible radio frequency identification tags (RFID) Organic FETs (OFETs) are viable candidates for flexible electronics because they are based on semiconducting p conjugated materials, including small molecules and polymers, which can be solution processed at low temperature on flexible substrates. Solution processing may enable the use of high throughput methods such as roll to roll coating and inkjet printing for low cost manufacturing. In the molecular perspective, the limitless ability to tune the properties of these materials just by a small modification of the conjugated backbone or sidechains makes them attractive to both academic research and industrial manufacturing. Between the two materials, semiconducting polymers offer better potential for the formation and mechanical properties of thin films, compared to their small molecule counterparts.For OFETs to be industrially viable, however, they must first have high charge carrier mobility. Recent advances in molecular designs and device engineering have seen significant increases in the mobility of OFETs fabricated with conjugated polymer (or PFETs) In this dissertation, it is shown that obtaining polymer chain alignment is critical to improve the mobility of PFETs. In addition, the charge transport mechanism is investigated to explain the high mobility in PFETs with aligned polymers. Most importantly, the mechanism of electrical instability and non ideality (i.e. the double slope) of high mobility PFETs with a certain degree of ambipolarity is unraveled. Firstly, the alignment of polymer chains inside polymer fiber bundles is revealed by high resolution atomic force microscopy (AFM) This alignment is enabled by nano grooves of ca. 50 nm wide and 2 5 nm deep on SiO2 substrates used for fabricating the PFETs. Mobility of charge transport along the direction of the polymer fiber is an order of magnitude higher than that of charge transport perpendicular to the fiber direction. It indicates that aligned polymer chains facilitate fast intrachain charge transport.Secondly, the charge transport mechanism is determined to be the thermally activated hopping of charge carriers. This is an important finding because it has been speculated that band transport is possible in OFETs fabricated from well aligned polymer fibers. With a normal range of molecular weight (30 kDa to 100 kDa) which is feasible for industrial scale up, the stretched length of the corresponding polymer chain, ranging from 50 150 nm, is not enough to cross the full channel length of the OFETs. In addition, polymer chains in a solution processed thin film are likely to have kinks and twists that disrupt the perfect electronic coupling necessary for band transport. It implies that polymers should be designed to facilitate not only chain alignment but also the strong electronic coupling in the p p stacking direction between chains for efficient hoppings between chains with low energetic barriers.Finally, the electrical instability and its associated non ideal device characteristics is thoroughly investigated. Electrical stability is as important as high charge carrier mobility for OFETs to be commercialized in large scale production. The use of low bandgap donor acceptor (D A) polymers for obtaining high mobility introduces undesirable electron current in p type PFETs. The effect of electron transport and trapping on hole conduction in p type OFETs has not been addressed. In this work, p type PFETs fabricated with SiO2 dielectrics and with a certain degree of ambipolarity exhibit electrical instability (a change in current upon bias stressing) and non ideality (double slope in transfer curves) of hole current. It is determined that electron trapping and the subsequent formation of SiO charges on SiO2/polymer interface are the principle origins of the instability and the double slope. Suppressing these undesired properties is essential to make PFETs industrial viable. It should be noted that the double slope has been a long debate without a solid explanation in the OFET research community.In sum, this thesis shows a comprehensive understanding of the structure processing property relationship of PFETs fabricated with well aligned polymers. New findings in the thesis provide important guidelines for molecular design and device engineering of high mobility and practical PFETs. These guidelines have been successfully demonstrated by us and our collaborators at UCSB.
author_names: Hung Phan
title: Molecular Assembly and Device Physics of High Mobility Organic Field Effect Transistor Fabricated from Donor Acceptor Copolymers
year: 2016

abstract: Intensive studies of conjugated molecules and polymers are carried out all over the world with the intent of obtaining cheap and efficient organic electronic devices. The most mature application at the moment is the light emitting diode, but also photovoltaic cells and different types of transistors shows promising results. Interest in these materials is based on possibilities of 'simple' and cheap processing techniques, comparing to inorganic compounds, in the manufacturing of devices. The understanding of the electronic and chemical structure of the surfaces and interfaces of these materials is a basic requirement for understanding the characteristics of the potential devices. Understanding the electronic structure of the pristine materials enables conclusions to be drawn concerning electrical and optical properties in these materials. The behaviour of the interface between metals and conjugated materials is one of the primary factors determining the suitability of using certain electrode/organic material combinations in device applications.With this motivation, the electronic structure of both conjugated molecules and polymers surfaces and their interfaces to metals (and insulators) have been studied with mainly photoelectron spectroscopy (PES) In some cases complementary techniques have been needed and performed. This includes the four point probe technique for determining surface resistance and atomic force morphology for determining surface morphology. As well as synchrotron based techniques, such as near edge X ray absorption spectroscopy and resonant photoemission have been used. The main results compromised in this thesis are summarized below.Poly(3,4 ethylenedioxythiophene) polystyrenesulfonic acid (PEDOT PSS) is an aqueous colloidal dispersion consisting of doped conjugated polymer PEDOT with counter ions provided by the PSS chains. PEDOT PSS films have previously proven to have a grain like structure in which the grains have a ~30 A thick insulating PSS outermost layer. The conductivity of thin PEDOT PSS films has been improved through adding some high boiling point solvents to the PEDOT PSS blend. The major reason for this increase is a rearrangement of the morphology, in terms of an increase in the PEDOT to PSS ratio in the surface region (i.e. the insulating PPS layer is decreased for each grain).The initial stages of interface of PEDOT PSS with aluminum for contacting purposes has also been examined. Due to the many components in the PEDOT PSS film its reactions with alurninum was difficult to deduce. Therefore the aluminum interfaces with model molecules of each of the components of PEDOT PSS were investigated to discern this. Phenyl capped EDOT trimer was used as a model oligomer for neutral PEDOT. It has been shown that aluminum preferentially interacts and forms covalent bonds with C S carbons that causes a rearrangement of the charge density within the oligomer and breaks then conjugation. In PEDOT PSS blends the PEDOT part is left intact and alurninurn preferentially reacts with the SO3 H+ and/or SO3 species of the PSS part.A specific blend of conjugated materials used in photovoltaic cells is a one to four mixture of APFO 3 (a low band gap copolymer based on alternating fluorene and donor acceptor donor units) to PCBM (soluble C60 derivative) The electrode systems studied are the widely used Al and Al/LiF contacts. We demonstrate a thickness dependent effect of the LiF layer in the Al/LiF/organic structure. LiF has a protective effect for all thickness preventing formation at the Al/organic interface of Al organic complexes that destroy the P conjugation. In addition to this, there are two other beneficial effects (depending on LiF thickness) Decomposition of LiF occurs for thin enough layers in which the LiF species are in contact with both the organic film and the A1 atoms. This results in Li doping of the organic films and creates a low workfunction contact. For thicker (multi)layers, the dipole formed at the LiP/organic interface is retained as no decomposition of the LiF occurs upon Al deposition.We have shown the occurrence of interfacial dipoles at C60/LiF/Al interfaces and confirmed interfacial dipoles at Alq3/Al, C60/Al and Alq3/LiF/Al interfaces through vacuum level shifts. There is strong interaction with the substrates in all cases. There is evidence of covalent interaction between both Alq3 and C60 films with the AI substrates. The added LiF layer (between AI substrate and the organic film) prevents the covalent bonds from forming and the LiF does not dissociate in any case, unlike what is found in literature for the reverse order of deposition. For both Alq3 and C60 there is charge transfer from the Al substrate to the organic film through the LiF layer. However, if the thickness of the LiF layer exceeds 25 A this charge transfer is blocked. The evolution of the electronic structure upon n doping of the first Alq3 monolayer observed here is different from previous studies of n doping mer Alq3, indicating that there is preferential deposition and/or formation of the unusual facial isomer of Alq3 on the LiF/Al substrate. Our results are the first reported photoemission spectra of this isomer and its n doped state.The electronic structure of two new low band gap polymers (APFO 3 and APFO 7) based on donor acceptor donor groups copolymerized with fluorine units has been characterized. The valence band of APFO 3 seems to be highly dispersed and derived from orbitals delocalized over the whole polymer chain, where as the conduction band is nearly flat as it is derived from orbitals localized on the acceptor units. The existence of a dispersed valence band would predict good hole transporting properties, where as a flat conduction band would be expected to produce poor electron transporting properties. The electronic structure of APFO 7 has similarities to APFO 7 but it is also less clear. The larger size of the acceptor unit seem to distort both the valence band and conduction band shape as compared to APFO 3, however, so further work is needed to understand the more complex APFO 7 system.
author_names: Stina Jonsson

William Raymond Salaneck

Mats Fahlman
title: X ray photoelectron spectroscopy study of the metal/polymer contacts involving aluminum and poly(3,4 ethylenedioxythiophene) poly(styrenesulfonic acid) derivatives
year: 2003

abstract: Intensive studies of conjugated molecules and polymers are carried out all over the world with the intent of obtaining cheap and efficient organic electronic devices. The most mature application at the moment is the light emitting diode, but also photovoltaic cells and different types of transistors shows promising results. Interest in these materials is based on possibilities of 'simple' and cheap processing techniques, comparing to inorganic compounds, in the manufacturing of devices. The understanding of the electronic and chemical structure of the surfaces and interfaces of these materials is a basic requirement for understanding the characteristics of the potential devices. Understanding the electronic structure of the pristine materials enables conclusions to be drawn concerning electrical and optical properties in these materials. The behaviour of the interface between metals and conjugated materials is one of the primary factors determining the suitability of using certain electrode/organic material combinations in device applications.With this motivation, the electronic structure of both conjugated molecules and polymers surfaces and their interfaces to metals (and insulators) have been studied with mainly photoelectron spectroscopy (PES) In some cases complementary techniques have been needed and performed. This includes the four point probe technique for determining surface resistance and atomic force morphology for determining surface morphology. As well as synchrotron based techniques, such as near edge X ray absorption spectroscopy and resonant photoemission have been used. The main results compromised in this thesis are summarized below.Poly(3,4 ethylenedioxythiophene) polystyrenesulfonic acid (PEDOT PSS) is an aqueous colloidal dispersion consisting of doped conjugated polymer PEDOT with counter ions provided by the PSS chains. PEDOT PSS films have previously proven to have a grain like structure in which the grains have a ~30 A thick insulating PSS outermost layer. The conductivity of thin PEDOT PSS films has been improved through adding some high boiling point solvents to the PEDOT PSS blend. The major reason for this increase is a rearrangement of the morphology, in terms of an increase in the PEDOT to PSS ratio in the surface region (i.e. the insulating PPS layer is decreased for each grain).The initial stages of interface of PEDOT PSS with aluminum for contacting purposes has also been examined. Due to the many components in the PEDOT PSS film its reactions with alurninum was difficult to deduce. Therefore the aluminum interfaces with model molecules of each of the components of PEDOT PSS were investigated to discern this. Phenyl capped EDOT trimer was used as a model oligomer for neutral PEDOT. It has been shown that aluminum preferentially interacts and forms covalent bonds with C S carbons that causes a rearrangement of the charge density within the oligomer and breaks then conjugation. In PEDOT PSS blends the PEDOT part is left intact and alurninurn preferentially reacts with the SO3 H+ and/or SO3 species of the PSS part.A specific blend of conjugated materials used in photovoltaic cells is a one to four mixture of APFO 3 (a low band gap copolymer based on alternating fluorene and donor acceptor donor units) to PCBM (soluble C60 derivative) The electrode systems studied are the widely used Al and Al/LiF contacts. We demonstrate a thickness dependent effect of the LiF layer in the Al/LiF/organic structure. LiF has a protective effect for all thickness preventing formation at the Al/organic interface of Al organic complexes that destroy the P conjugation. In addition to this, there are two other beneficial effects (depending on LiF thickness) Decomposition of LiF occurs for thin enough layers in which the LiF species are in contact with both the organic film and the A1 atoms. This results in Li doping of the organic films and creates a low workfunction contact. For thicker (multi)layers, the dipole formed at the LiP/organic interface is retained as no decomposition of the LiF occurs upon Al deposition.We have shown the occurrence of interfacial dipoles at C60/LiF/Al interfaces and confirmed interfacial dipoles at Alq3/Al, C60/Al and Alq3/LiF/Al interfaces through vacuum level shifts. There is strong interaction with the substrates in all cases. There is evidence of covalent interaction between both Alq3 and C60 films with the AI substrates. The added LiF layer (between AI substrate and the organic film) prevents the covalent bonds from forming and the LiF does not dissociate in any case, unlike what is found in literature for the reverse order of deposition. For both Alq3 and C60 there is charge transfer from the Al substrate to the organic film through the LiF layer. However, if the thickness of the LiF layer exceeds 25 A this charge transfer is blocked. The evolution of the electronic structure upon n doping of the first Alq3 monolayer observed here is different from previous studies of n doping mer Alq3, indicating that there is preferential deposition and/or formation of the unusual facial isomer of Alq3 on the LiF/Al substrate. Our results are the first reported photoemission spectra of this isomer and its n doped state.The electronic structure of two new low band gap polymers (APFO 3 and APFO 7) based on donor acceptor donor groups copolymerized with fluorine units has been characterized. The valence band of APFO 3 seems to be highly dispersed and derived from orbitals delocalized over the whole polymer chain, where as the conduction band is nearly flat as it is derived from orbitals localized on the acceptor units. The existence of a dispersed valence band would predict good hole transporting properties, where as a flat conduction band would be expected to produce poor electron transporting properties. The electronic structure of APFO 7 has similarities to APFO 7 but it is also less clear. The larger size of the acceptor unit seem to distort both the valence band and conduction band shape as compared to APFO 3, however, so further work is needed to understand the more complex APFO 7 system.
author_names: Stina Jonsson

Elin Carlegrim

Fengling Zhang

William Raymond Salaneck

Mats Fahlman
title: Photoelectron spectroscopy of the contact between the cathode and the active layers in plastic solar cells the role of LiF
year: 2005

abstract: At the nanoscale measures can move from a mass scale analogue calibration to counters of discrete units. The shift redefines the possible levels of control that can be achieved in a system if adequate selectivity can be imposed. As an example as ionic substances pass through nanoscale pores, the quantity of ions is low enough that the pore can contain either negative or positive ions. Yet precise control over this selectivity still raises difficulties. In this issue researchers address the challenge of how to regulate the ionic selectivity of negative and positive charges with the use of an external charge. The approach may be useful for controlling the behaviour, properties and chemical composition of liquids and has possible technical applications for nanofluidic field effect transistors [1] Selectivity is a critical advantage in the administration of drugs. Nanoparticles functionalized with targeting moieties can allow delivery of anti cancer drugs to tumour cells, whilst avoiding healthy cells and hence reducing some of the debilitating side effects of cancer treatments [2] Researchers in Belarus and the US developed a new theranostic approach combining therapy and diagnosis to support the evident benefits of cellular selectivity that can be achieved when nanoparticles are applied in medicine [3] Their process uses nanobubbles of photothermal vapour, referred to as plasmonic nanobubbles, generated by plasmonic excitations in gold nanoparticles conjugated to diagnosis specific antibodies. The intracellular plasmonic nanobubbles are controlled by laser fluence so that the response can be tuned in individual living cells. Lower fluence allows non invasive high sensitive imaging for diagnosis and higher fluence can disrupt the cellular membrane for treatments. The selective response of carbon nanotubes to different gases has leant them to be used within various different types of sensors, as summarized in a review by researchers at the University of California, Riverside [4] Mangu et al in the US have developed highly sensitive and selective room temperature gas sensors made from composites of multiwalled carbon nanotubes and polymers [5] They report sensitivities as high as 28% when exposed to 100 ppm of NH(3) and 29.8% to 100 ppm of NO(2) Nanopore structures are also showing increasing promise for sensing and biophysical characterization applications, in particular DNA [6] An applied potential drives negatively charged DNA molecules through nanopores in a membrane and gives rise to current blockage pulses that are characteristic of specific analytes. Solid state nanopore structures hold advantages over biological pores, such as those in a haemolysin protein, as they are more resilient to experimental conditions, and ideally should also allow control of the nanopore diameter, channel length and surface composition. Asghar and colleagues have now reported a method that enables just that, 'a rapid solid state nanopore fabrication and controlled pore shrinking process which does provide simultaneous in situ control of surface properties' [7] In addition, they demonstrate the viability of the approach for single molecule sensor applications using double stranded DNA. In this issue, researchers in China report on a different approach which allows control over the transport of ionic fluids through nanopore type structures. They describe the rapid field effect control of electrical conductance in single nanotube nanofluidic transistors [1] Rather than seeking to control the charge of thenanotube inner surface, Gong and colleagues control polarity switching based on negative and positive ion selectivity using an external charge. 'The polarity of the nanotube can be reversed and tuned by the external field, which could find interesting applications in the field of ion separation and energy conversion' they explain, adding that the system may also find a use as a voltage sensor through the detection of the type of ions across the channel. The aim of achieving selectivity encompasses a huge range of fields in nanotechnology research, from sensing and medicine to nanoelectronics and self assembly. As our understanding of how nanosystems behave deepens, so too does the hunger to improve our capabilities, allowing greater precision and control in manipulating these systems. Selectivity is far from trivial when shrinking to systems of nanoscale dimensions, but the range of opportunities it brings just keeps on growing. References [1] Gong X, Li J, Guo C, Xu K and Hui Y 2012 Molecular switch for tuning ions across nanopores by an external electric field Nanotechnology 24 025502 [2] Brannon Peppas L and Blanchette J O 2004 Nanoparticle and targeted systems for cancer therapy Adv. Drug Deliv. Rev 56 1649 59 [3] Lukianova Hleb E Y, Hanna E Y, Hafner J H and Lapotko D O 2010 Tunable plasmonic nanobubbles for cell theranostics Nanotechnology 21 085102 [4] Zhang T, Mubeen S, Myung N V and Deshusses M A 2008 Recent progress in carbon nanotube based gas sensors Nanotechnology 19 332001 [5] Mangu R, Rajaputra S and Singh V P 2011 MWCNT polymer composites as highly sensitive and selective room temperature gas sensors Nanotechnology 22 215502 [6]Meller A, Nivon L, Brandin E, Golovchenko J and Branton D 2000 Rapid nanopore discrimination between single polynucleotide molecules Proc. Natl Acad. Sci. 97 1079 84 [7] Asghar W, Ilyas A, Deshmukh R R, Sumitsawan S, Timmons R B and Iqbal S M 2011 Pulsed plasma polymerization for controlling shrinkage and surface composition of nanopores Nanotechnology 22 285304.
author_names: Anna Demming
title: Nanotechnological selection.
venue: Nanotechnology
year: 2013