Li-Hsin Chan

Blue Light-Emitting Devices Based on Molecular Glass Materials of Tetraphenylsilane Compounds**

Only the full gap, at 2 lm, is shaded along the whole Brillouin zone. We can establish a clear correspondence between all the peaks observed experimentally and the calculated PBGs along C‐L. This comparison indicates that the third reflectance peak (near k = 2.1 lm, as indicated with an asterisk) in the black line spectrum in Figure 4 corresponds to a full PBG in the inverted opal. Good agreement is also found between the theoretical and experimental full photonic gap to mid-gap ratio, being 8.6 % and 6.5 %, respectively. As light impinges on the samples at angles slightly deviated from the C‐L direction, reflectance peak widths and positions are slightly different from those extracted from Figure 5. Although the results shown here indicate the opening of a full PBG in the Ge inverse opal, different kinds of disorder might affect it, and a thorough optical analysis of all crystalline directions should be done to confirm it. This is the focus of future research. In summary, we have made germanium inverse opals by a CVD technique. By varying the synthesis conditions, the degree of germanium infiltration can be controlled, which enables the optical properties of the materials to be engineered. The optical characterization of the materials confirms their PBG behavior in the near infrared region. Good agreement has been found between theory and experiment which indicates that a full PBG has opened up near 2 lm.

Non-doped red organic light-emitting diodes

A convenient and improved procedure has been developed for preparing the red fluorophore N-methyl-bis(4-(N-(1-naphthyl)-N-phenylamino)phenyl)maleimide (NPAMLMe) through the efficiently synthesized bis(4-bromophenyl)fumaronitrile, the necessary precursor in preparing NPAMLMe. This allows NPAMLMe to be an easily accessible material compared with other known red, organic light-emitting diode (OLED) materials. We also report an unusual approach in fabricating red OLEDs, which does not adopt a conventional red dopant but rather NPAMLMe as the host red emitter. The performance of the non-doped red devices has been studied in depth for the first time. Devices with varied layer thickness were fabricated for examining the compatibility of NPAMLMe with commonly known materials, electron-transporting tris(8-hydroxyquinolinolato)aluminium (Alq3) and hole-transporting 4,4′-bis(4-(N-(1-naphthyl)-N-phenylamino)phenyl)biphenyl (NPB). In the presence of a hole-blocking layer of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), the devices emit pure red electroluminescence (EL), and it is essentially voltage-independent. Red EL with a brightness near 4600 cd m−2 and an external quantum efficiency as high as 1.6% has been achieved. The performance of such non-doped, red OLEDs is comparable with or better than contemporary, dopant-based, red OLEDs, and the simple fabrication is the advantage of the approach.

From crystals to columnar liquid crystal phases: molecular design, synthesis and phase structure characterization of a series of novel phenazines potentially useful in photovoltaic applications

It is known that in photovoltaic applications, columnar discotic liquid crystal (LC) phases of conjugated compounds are useful to align the molecules for improving their charge mobilities. However, conjugated compounds are usually either crystalline or amorphous. For compounds to form columnar discotic LC phases, specific molecular design is required for their ordered structural packing. In our recent report, a series of conjugated compounds, 6,7,15,16-tetrakis(alkylthio)quinoxalino-[2′,3′:9,10]-phenanthro[4,5-abc]phenazine (TQPP-[SCn]4) (n = 6, 8, 10 and 12), which display p-channel characteristics, were synthesized and characterized. This series of compounds was crystalline and did not exhibit LC behavior (S. Leng, B. Wex, L. H. Chan, M. J. Graham, S. Jin, A. J. Jing, K.-U. Jeong, R. M. Van Horn, B. Sun, M. Zhu, B. R. Kaafarani and S. Z. D. Cheng, J. Phys. Chem. B, 2009, 113, 5403–5411). In order to create a columnar LC phase with the lowest free energy within a broad applicable temperature region, we specifically designed and synthesized several series of electron-deficient phenazine derivatives to disrupt the molecular crystal packing and force the compounds to enter the columnar LC phase. These phenazine derivatives were designed to control the fused rigid ring size and shape as well as the location, lengths, and chemical structures of their flexible tails. These series include a series of 2,11-bis(1-methylethyl)-6,7,15,16-tetrakis(alkoxy)quinoxalino[2′,3′:9,10]phenanthro-[4,5-abc]-phenazines (TQPP-[t-Bu]2-[OR(B)]4), a series of 2,13-bis(1-methylethyl)-7,8,18,19-tetrakis(alkoxy)pyrazino[2,3-i]pyrazino[2″,3″:6′,7′]quinoxalino[2′,3′:9,10]phenanthro[4,5-abc]-phenazines (TPPQPP-[t-Bu]2-[OR(B)]4), and a series of 3,4,11,12,19,20-hexaalkoxy-2,5,7,8,10,13,15,16,18,21,23,24-dodecaazatri-anthracenes (HDATAN-[OR]6), where R is the alkyl chain in the substituents and B represents that they are branched structures. The different phase structures and transition behaviors of these series of compounds were studied, and based on the experimental results, we can conclude that tailoring the alkyl tail size, the core size, and the core shape leads to a promising way to design molecules that exhibit the columnar LC phase. In particular, changes in alkyl tail architecture affect the phase behaviors more significantly than changes in its length.

Sequential self-repetitive reaction toward wholly aromatic polyimides with highly stable optical nonlinearity

A sequential self-repetitive reaction (SSRR) based on carbodiimide (CDI) chemistry was utilized for preparing a high-yield wholly aromatic polyimide. The polyimide was synthesized with 4,4′-methylene-diphenylisocyanate (MDI) and a di(acid-ester) compound which was derived from the ring-opening reaction of 3,3′,4,4′-oxydiphthalic dianhydride (ODPA) at room temperature by the addition of equimolar methanol. Poly-CDI was first synthesized from MDI. The di(acid-ester) compound was then reacted with poly-CDI to form poly(N-acylurea). After curing process, N-acylurea moiety was converted to di(ester-amide) structure viaSSRR and further subjected to a ring-closure reaction to form the wholly aromatic polyimide with a Tg of 247 °C. This approach was further taken to prepare thermally stable nonlinear optical (NLO) materials. Similarly a diimide-diacid containing chromophore was reacted with poly-CDI to obtain an intermediate, poly(N-acylurea). The poly(N-acylurea) with the ester side groups would exhibit excellent organosolubility, which enabled the fabrication of high quality optical thin films. After in situ poling and curing processes, N-acylurea moiety was converted to di(ester-amide) structure viaSSRR and further subjected to a ring-closure reaction to form the wholly aromatic NLO polyimide with an electro-optical coefficient, r33 of 25 pm/V (830 nm). Excellent temporal stability at elevated temperatures (200 °C) and a waveguide optical loss of 2.5 dB cm−1 at 1310 nm were also obtained.

The crystalline properties of carbon nitride nanotubes synthesized by electron cyclotron resonance plasma

Abstract Carbon nitride nanotubes (CN-NT) have been synthesized by an electron cyclotron resonance chemical vapor deposition (ECR–CVD) system with a mixture of C 2 H 2 and N 2 as precursors without using any catalyst. The carbon nitride nanotubes were synthesized in an anodic alumina membrane as template in which a packed array of parallel, straight and uniform channels with a diameter of approximately 50-nm and 30-μm thick exists. Samples were analyzed by field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Preliminary results showed that the properties of the deposited carbon nitride nanostructures depend on process parameters, such as deposited temperature, ratio of the precursors and microwave power. The aligned nanostructures have been verified by FESEM and HRTEM. The FTIR spectra reveal that some of the carbon atoms may have possibly been substituted by oxygen atom in the carbon nitride nanotubes. From the XPS results, N is either bonded to two C atoms (sp 2 pyridine-like type) or to three (sp 3 urotropine-like type) in the hexagonal sheets. The Raman spectrum showed that carbon nitride nanotubes have a high degree of graphitization.

The effect of argon on the electron field emission properties of α-C:N thin films

Abstract Amorphous carbon nitride (α-C:N) thin films were synthesized on silicon as electron emitters by the electron cyclotron resonance chemical vapor deposition (ECR-CVD) system in which a negative dc bias was applied to the graphite substrate holder and a mixture of C2H2 and N2 was used as precursors. The addition of Ar combined with the application of a negative dc bias can increase nitrogen content (N/C) measured by X-ray photoelectron spectroscopy (XPS), eliminate the dangling bonds in the film determined by Fourier transform infrared (FTIR) spectroscopy, decrease the film thickness measured by field emission scanning electron microscope (FE-SEM), increase the film roughness measured by atomic force microscope (AFM) and raise the graphitic content examined by Raman spectroscopy. The result shows that the onset emission field of α-C:N with Ar addition to the precursors can be as low as 4.5 V μm−1 compared with 9.5 V μm−1 of the film without the addition of Ar.

Effect of bias voltage on the formation of a-C:N nanostructures in ECR plasmas ☆

Amorphous carbon nitride (a-C:N) nanotubes and nanofibers on porous alumina templates were synthesized by an electron cyclotron resonance chemical vapor deposition system in which a variable negative d.c. bias was applied to the substrate holder of graphite to promote the flow of ionic fluxes through the nano-channels of the alumina template in microwave excited plasmas of C2H2 or N2. The aligned structures of a-C:N nanotubes or nanofibers were verified by field emission scanning electron microscopy. Transmission electron microscopy micrographs showed that a-C:N nanotubes and nanofibers were the size with a diameter of ∼100–250 nm and a length of ∼50–80 μm. The amorphous nature of the nanostructures was confirmed by the absence of crystalline phases arising from selected area diffraction patterns. X-ray photoelectron spectroscopy spectra indicated that a-C:N nanotubes and nanofibers were composed of nitrogen and carbon, and the N/C ratios could reach as high as 72%. The absorption bands between 1250 and 1750 cm−1 in Fourier transform infrared spectroscopy provided direct evidence for the presence of nitrogen atoms in the amorphous carbon network. The well-aligned a-C:N nanotubes and nanofibers are expected to have potential applications in optical, electronic and optoelectronic devices.

Optimization of Tetraphenylsilane-Based Blue Organic Light-Emitting Devices with Copper Phthalocyanine

Multilayer organic light-emitting devices emitting with blue elelctroluminescence were fabricated. Devices with and whithout a layer of copper phthalocyanine (CuPc) as hole injection material were studied. / (current density)-V (voltage)-L (luminance) characteristics and electroluminescence efficiency of both devices were compared.

Nondoping red fluorophores for red organic light-emitting diodes

One of the first bright (~8,000 cd/m2 of maximum electroluminance) and efficient (2.4% of maximum external quantum efficiency) saturated red (coordinates x = 0.66, y = 0.32 of 1931 CIE chromaticity) nondopign OLEDs has been achieved based on novel N-Methyl-bis(4-(N-(1-naphyl)-N-phenylamino)phenyl)maleimide red fluorophore. The developing process in achieving the high performance of the device is presented.

Bottom-Contact n-Channel Organic Thin-Film Transistors with Naphthalene-Based Derivatives

We investigated bottom-contact organic thin-film transistors based on n-benzyl naphthalene 1,4,5,8-tetracarboxylic diimides. The electrical characteristics were tested in both air and vacuum environments. n-Type semiconductors which are modified by fluorinated imide can be operated in ambient environment. The close packing of the fluorinated ester imide group leads to carrier mobility as high as 1.6 X 10 -2 cm 2 V -1 s -1 . The short distance between each molecule can be found via a single-crystal structure.