We consider that the effects of exchange surface ligand on the performance of hybrid organic/inorganic light emitting diodes ( LED) that use the colloidal Nano- crystal quantum dots as emissive centers. By using the series of main alkylamine with different chain of lengths, by exchange the native ligands surface with a series of CdSe/ZnS /CdZnS core/shell shell nanocrystal quantum dots and checked the results in photoluminescence and electroluminescence efficiency of emissive quantum dot layer. By fabricate LEDs with made butylamine, octadecylamine, and octylamine exchanged quantum dots. We find the differences in photoluminescence efficiency of the quantum dots are not always proportional to electroluminescence efficiency of the devices.from the use of nanoparticales competing both needs of high photoluminescence efficiency and good injection charge and transfer energy.
Quantumdots or semiconductor Nan crystals are solution-process able chromosphores with size-tunable band gaps and high photoluminescence quantum efficiencies. with comparing most organic chromophores they have excellent phtostability, narrow emission line width greater then 30nm, and larger spin orbit coupling. Like wise photodiode these factors make them for use in solution processable thin film optoelectronic devices.[1,2] and light emitting diodes (LED).The quantum dots subjugated good external quantum efficiency (1-2%) with high brightness(less then 6000cd * m/2), low drive voltage (~6.5V at 100cd * m/2),narrow electroluminescence spectra(greater then 30nm)[3]. However the result of electroluminescence from excitones directly formed by electrical injection of carriers into the quantum dots or on the organic hosts formed predominantly the excitions before undergoing the resonant energy transfer to emitter of quantum dot. We don't know which affect energy transfer or injection is dominant during device operation. Through controlling of the quantum dot surface ligands it is the way to modulate the interactions between the organic transport layers and the quantum dots. Approximately 10 years before Greenham and Alivisators was proved that a cating of tri-n-octylphosphine oxide was able to severely photo induced electron transfer [4] from a conjugated polymer to CdSe nanocrystal. The alkyl side chains on semiconducting polymer could inhibit transfer charge in quantum dots and polymer together explained by Ginger and Greenham [2]. In advanced the Sargent group was proved that the photoconductive performance of polymer /pbS quantum dot photodiodes could be improved by two orders of magnitude through control of the ligand surface barrier[5]. Mattoussi [6] was discovered the overcoating of CdSe nanocrystel with inorganic ZnS shells results brighter quantum dots with higher photoluminescence quantum yields but such dots performed less luminescent uncoated CdSe cores in QD-LED structure. Due to the wide band gap shell this effect to increased difficulty of carrier injection into the core shell nanocrystals structure, now we will see the length of the organic ligands surface on the device performance of colloidal semiconductor quantum dots, and compare electroluminescence and photoluminescence series of QD-LED, in which exchanged quantum dots have had their surface ligands with octadecylamine,octylamine,and butylamine ligands and see the results in terms of nanocrystal film morphology.
Introduction:
By means of nanocrystal defined as material having a crystal size at the nano-scale stage and consists a number of hundred to a number of thousand atoms. In view of the fact that the small sized nanocrystal has a large surface area per unit volume, the majority of atom establishing the nanocrystal are nearby at the surface of the nanocrystal. Based on this structure, the nanocrystal shows quantum confinement effect and shows electrical, magnetic, optical, chemical and mechanical properties different from those attribute to the basic atoms of the nanocrystal. That is control over the physical size of the nanocrystal enables the control of several properties.
Organic light emitting diode (OLED) is a semiconductor device capable of converting electrical energy into light energy with a high exchanging efficiency. Organic light emitting diodes, also known as organic electroluminescent devices, include an anode, a cathode and an electroluminescent medium made up of tremendously thin layers distinguishing the anode and the cathode. The organic light emitting diode is a photoelectric device and can convert the electric energy into optical form in high converting efficiency and is frequently used in an signifying lamp or a displaying board. OLED displays have advantages of light emission, high incandescent efficiency, wide viewing angle, fast response speed, high reliability, full color, low-voltage drive, low power consumption, and simple fabrication.
All semiconductors are focus to optical charge carrier generation. This is typically an undesired effect, so the majority semiconductors are packaged in light blocking material. Photodiodes are proposed to sense light(photo detector), so they are packaged in materials that allow light to pass, and are more often than not PIN (the kind of diode most sensitive to light). A photodiode can be used in solar cells, in photometry, or in optical communications. Several photodiodes may be packaged in a single device, both as a linear array or as a two-dimensional array. These arrays should not be puzzled with charge-coupled devices. [7]
Aim.
Aim of the Fabrication of nanostructures and their application is to developed versatile understanding and non destructive fabrication method and explore their properties and potential of their application in electronic devices like as photodiode.
The first aim or objective is the development of atom lithography and etching techniques of nano technologies based on the deposition of atom focused by laser light.
Cd and Se, or Zn and S which will open an innovative approach based on the integration with well established deposition methods. Growth control at the atomic scale, regular spatial ordering, superior spatial definition, parallel operation, will be exploited in the fabrication of basic building blocks for innovative devices, to be used in space resolved photoemission, single electron devices, photonic crystals. Detailed experimental and theoretical analyses will be carried out in order to define advantages and limits of our process in view of industrial applications.
Objectives in nanotechnology enables sensor are fabricated from Nan particles, one dimensional nano materials, thin film of nano scale thickness and or thin film comprised of nano structures.
The nano structure can be used both in the fabrication of sensing layer and or the fabrication of transducer structures .some of nano/ micro fabrication technologies are very mature and widely used, such as photo- lithography, whilst other are in their infancy falling into naching application.
Knowledge of nano materials synthesis nano structures thin film deposition and nano pattering technique is fundamental of the development of nano technology enabled sensor and their sensitive layer.Another the driving force of nano fabrication and nano pattering is that the dimension of transcducer which are fabricated using standard micro technology techniques is becoming smaller and smaller.
They are rapidly approaching nano dimension feature size. At such scale unique technology for synthesis of nano material and nano pattering technique is imperative for the fabrication of transducer structure and their connection to
Objectives.
To searched and developed a emitting of white light Electroluminescence device comprised misconduct nanocrystals and we have found that a device of light emitting inorganic and organic hybrid electroluminescence where at the same time semiconductor nanocrystal layer, an electron transport layer and a hole transport layer light emit and achieved emission of white light or different sizes and constitution of semiconductor nanocrystal at different wavelengths simultaneously light emit and achieved emission of white light. At maximum brightness performing the lowest EQE of any quantum dot film QD-LEDs with butylamine starts the highest of all three devices (at low current/brightness) and then drops fastest with increasing current density, This development, particularly the high efficiency at low brightness, is particularly interesting given the comparatively low photoluminescence efficiency of the butylamine-exchanged quantum dots (~25% that of the octadecylamine quantum dots).As compared to photoluminescence efficiency , the electroluminescence efficiency is clearly not a simple function for these quantum dots. CdSe/ZnS core/shell quantum dots in QD-Elsewhere the increased photoluminescence efficiency of quantum dots with thicker shells translated into insignificant gains in electroluminescence performance because the thick shell decreases the possibility of forming an exciton on the core/shell particles [8].
Project Outline.
Fabrication of Nanostructures and their application in Photodiodes
CdSe/ZnS (560nm)
Specifications of the CdSe/ZnS (560nm) are shown below
Description:
Yellow emitting quantum dot nanoparticales have dispersion form.
5 mglmL in toluene concentration
λex 545 nm; λem 560 nm, FWHM <40 nm, quantum yield 30‑50% have fluorescence.
stabilized with hexadecylamine (HDA) ligand coating surface treatment have matrix active group.
3.4 nm nano particle size
110 °C boiling point..
4 °C have flash point. [9]
Absorption extinction/ 0.97 x 10-5 M-1cm-1
Work Description.
Ligand exchange and synthesis of Nanocrystal
CdSe quantum dots with a core diameter of 2.9nm were synthesized by the CdO precursor route of Li et al. in the existence of oleic acid, octadecylamine, trioctylphosphine oxide, and octadecene [10].
Afterward inorganic shell of Cd (0.5) Zn (0.5) S ZnS was grown by adapting the consecutive ion layer adsorption reaction method of Li etal. To grow the first shell monolayer 0.7ml of a mixed 0.02M Cd-oleate and 0.02 MZn-oleate precursor solutions was added. The second shell layer was grown using 1ml of a 0.04 Zn-oleate precursor solution .After growth of the CdZnS and ZnS shells, the particles were collected by rainfall with acetone. The quantum dots are then dehydrated under nitrogen and stored in the dark until use. For the ligand replace experiments a single batch of core/shell/shell quantum dots was divided into three centrifuge tubes. The quantum dots in each tube ware washed by suspending the quantum dot in a few drops of reagent grade chloroform followed by the addition of ~8ml of acetone and mixing well. The quantum dots were precipitated from solution of centrifuging for 5 minutes at 3000rpm and were subsequently dehydrated on a schlock line under nitrogen gas. After that an excess of whichever 100micro liter of butylamine, 200micro meter of octylamine or 100mg of octylamine has added to each vial and reagent grade chloroform was added to top off the solution so that their volumes were even. The solutions of quantum dots in amines were shaken and permitted to sit at least 5 minutes. The quantum dots were then washed two extra time by suspended the quantum dots in chloroform and precipitating with acetone followed by ventilation under nitrogen gas. the centrifugation time compulsory to assemble the quantum dot precipitate from the solutions for the period of the washing steps varied plainly for the quantum dots passivated with each ligand, with butylamine exchanged dots precipitating the fastest, followed by octylamine exchanged dots, followed by octadecylamine passivated dots for the first washing after exchange.
Fabrication of Device
LEDs were fabricated Multilayer quantum dot-organic using methods derived since we have earlier explained [11]. The first step was the spin-coating of a cross linkable
HTL, BTPD-VB, onto a plasma-cleaned indium-tin oxide (ITO) covered slide. Each BTPD-VB molecule contained two N, N0-diphenyl-N, N0-(3 methyl phenyl) [1, 10biphenyl]-4, 40-diamine (TPD) groups connected through an ether linkage, each TPD group is bound to a cross linking group (4-vinylbenzylether), so that each molecule contains a 1:1 ratio of cross linker to TPD. The BTPD-VB layers were thermally cross-linked at 150 degree centigrade for 30-40 minutes. A second cross linkable HTL, bis (vinylbenzylether) -4, 40,400-tris (N-carbazolyl) diphenylamine (BiVB-TCTA), be after that spin-coated on top of the cross linked n BTPD-VB layer and cross-linked at 180 C for 30 minutes. Mutually thermal cross-linking steps were performing under argon. Earlier, we have using two HTLs in the QD-LED device leads to enhanced performance compare to a single-HTL device structure [11]. Next, the quantum dot layer was formed by spin-coating a chloroform solution containing quantum dots with an optical density of nearly 2 at a speed of 1500 rpm, thus forming a thin monolayer of quantum dots on the surface of the BiVB-TCTA film. A layer of the electron-transporting material, 2, 20,200-(1, 3,5
Benzenetriyl) tris[1-phenyl-1H-bezimidazole] (TPBI) (50 nm), [18,19] was then deposit on top of the quantum dot layer by means of thermal evaporation. In device fabrication the final step was the thermal evaporation of the cathode (30 nm of Ca, capped by 120 nm of Ag), which was deposit through a shadow mask. All thermal evaporation occurred under a vacuum of less than 1*106 Torr. The complete device structure is depicted in Fig.
Photoluminescence measurements
Explanation photoluminescence quantum yield measurements of the quantum dots were calibrated by means of rhodamine 101 as a standard (100% quantum yield in ethanol with an excitation wavelength of 560 nm, by using a Luminescence Spectrometer Perkins Elmer LS50B. The response of detector of the spectrometer was calibrating with an Ocean Optics LS-1-Cal calibrated light source. at the excitation wavelength of the solution absorbance has measured on an Agilent 8453 UV-vis spectrometer. The relative photoluminescence measurements of quantum dots thin films inside the QD-LEDs has performed in epi-fluorescence form on an reversed Nikon 2000U microscope in a closed cell purged with N2(g) by means of a filtered halogen lamp as the excitation source. The excitation and emission light were separated with suitable bandpass, dichroic, and long pass filters (excitation centered at 480nm). The photoluminescence be after that passed through a 560nm long pass filter and going to an Ocean Optics USB2000 charge-coupled device (CCD) spectrometer using a fiber optic cable. The quantum dot thin film photoluminescence spectra were not correct for the complete response of the system, but as alternative relative measurements suitable for comparing differences between samples. [12]
Electroluminescence measurements
The electroluminescence spectra of the QD-LEDs were recorded with an Oriel Intraspec IV CCD camera. The current-voltage characteristics of each 3.14mm2-pixel were measured on a Hewlett Packard 4155B semiconductor parameter analyzer. A Newport 2835-C multifunction optical meter was used to measure the electroluminescence
emission power. All electroluminescence and current- voltage (IV) measurements were performed in air. As a result, device performance was observed to degrade after only a few minutes operation at high current densities. [12]
Electroluminescence of Colloidal CdSe quantum dot
Non-contact form of AFM topography images of thin films of (a) octadecylamine (RMS roughness 1.1 nm), (b) octylamine (RMS roughness 1.1 nm), and (c) butylamine (RMS roughness 2.1 nm) passivated quantum dots spin-coated onto the organic hole transport layer In the Lab by making LEDs using CdSe/ZnS (560nm) nanoparticals
By means of making LED or photodiode using CdSe/ZnS (560nm) material for completing the process I have done the following Steps,
For purpose of heating process I put the thermocouple on the hot plate about 200C to 250C.
Take ITO (Silicon glass) in copper mask.
Before placing washed with acetone and then isopropyl alcohol and sonncate and then dry it.
Checked the conductive side and place in mask in that direction where the conductive side is in down word, titer the screwed and once again checked the conductive side.
Placed one drop of P-dot on active side or conductive side.
Started spin coater for 3 minutes with 180 rps.
Dried for 30 minutes on hot plate.
Cooled at room temperature and put in spin coater again.
Put 2 to 3 drops of acting or selected material of CdSe/ZnS (560nm).
Spin again at 1000 rps for 30 seconds approximately.
Dried made a board like a structure of molybdenum.
Put in the physical vapor deposition which has low voltage but very high current operation of CVD (chemical vapor deposition).
Started by pressing inter current the current by knob maintain the pressure up to 1*1/1000000 mbr.
Increased the temperature when the color of molybdenum bright red.
Released the pressure hang the copper mask in the vacuum chamber place the aluminum in the board started the vacuum chamber and wait until 1/1000000 mbr.
Heated the aluminum board.
At the end stop heating and cool down stop the pump take the sample.
Screeched the one end of sample to made anode connected the wires to anode and the entire cathode and fixed with silver and put gel on the edges for strength and checked the results of diode on oscilloscope. The collected data are as under
Test No Time Current Light Intensity (Lux) Voltage
0.000000 0.000000 0.000010 0.300000 0.003221
1.000000 300.000000 0.000050 0.300000 0.016076
2.000000 600.000000 0.000090 0.300000 0.028876
3.000000 900.000000 0.000130 0.300000 0.041916
4.000000 1200.000000 0.000170 0.300000 0.054705
5.000000 1500.000000 0.000210 0.300000 0.067540
6.000000 1800.000000 0.000249 0.300000 0.079991
7.000000 2100.000000 0.000289 0.300000 0.092776
8.000000 2400.000000 0.000329 0.300000 0.105459
9.000000 2700.000000 0.000369 0.300000 0.118054
10.000000 3000.000000 0.000409 0.300000 0.130511
11.000000 3300.000000 0.000449 0.300000 0.143276
12.000000 3600.000000 0.000489 0.300000 0.155735
13.000000 3900.000000 0.000529 0.300000 0.168183
14.000000 4200.000000 0.000569 0.300000 0.180475
15.000000 4500.000000 0.000609 0.300000 0.192964
16.000000 4800.000000 0.000648 0.300000 0.204918
17.000000 5100.000000 0.000688 0.300000 0.217080
18.000000 5400.000000 0.000728 0.300000 0.229127
19.000000 5700.000000 0.000768 0.300000 0.241498
20.000000 6000.000000 0.000808 0.300000 0.253192
21.000000 6300.000000 0.000848 0.300000 0.265226
22.000000 6600.000000 0.000888 0.300000 0.277545
23.000000 6900.000000 0.000928 0.300000 0.289284
24.000000 7200.000000 0.000968 0.300000 0.301033
25.000000 7500.000000 0.001008 0.300000 0.315461
26.000000 7800.000000 0.001047 0.400000 0.326489
27.000000 8100.000000 0.001087 0.300000 0.338123
28.000000 8400.000000 0.001127 0.300000 0.349398
29.000000 8700.000000 0.001167 0.300000 0.360753
30.000000 9000.000000 0.001207 0.300000 0.371772
31.000000 9300.000000 0.001247 0.400000 0.382907
32.000000 9600.000000 0.001287 0.300000 0.394354
33.000000 9900.000000 0.001327 0.300000 0.405017
34.000000 10200.000000 0.001367 0.300000 0.415907
35.000000 10500.000000 0.001407 0.300000 0.426770
36.000000 10800.000000 0.001446 0.300000 0.437274
37.000000 11100.000000 0.001486 0.300000 0.447599
38.000000 11400.000000 0.001526 0.300000 0.458255
39.000000 11700.000000 0.001566 0.400000 0.468870
40.000000 12000.000000 0.001606 0.300000 0.479492
41.000000 12300.000000 0.001646 0.300000 0.489611
42.000000 12600.000000 0.001686 0.300000 0.499673
43.000000 12900.000000 0.001726 0.300000 0.509956
44.000000 13200.000000 0.001766 0.300000 0.520261
45.000000 13500.000000 0.001806 0.300000 0.530045
46.000000 13800.000000 0.001845 0.300000 0.539811
47.000000 14100.000000 0.001885 0.300000 0.549576
48.000000 14400.000000 0.001925 0.300000 0.559646
49.000000 14700.000000 0.001965 0.300000 0.570342
50.000000 15000.000000 0.002005 0.300000 0.580283
Conclusions
By synthesize CdSe/CdZnS/ZnS core/shell/shell quantum dots and integrated them into hybrid QD-LEDs by means of cross-linked organic HTLs. later than exchanging the quantum dot surface ligands with octadecylamine, octylamine, and butylamine observed that a decrease in the quantum dot photoluminescence efficiency (both in solution and in thin films) by a factor of ~2 and ~ 4, correspondingly for the octylamineand
butylamine-exchanged particles comparing with the octadecylamine-exchanged quantum dots. The device characteristics of the QD-LEDs incorporated butylamine-exchanged particles are extensively different from those of QD-LEDs prepared with the other two types of ligand-exchanged particles. Also observed that the electroluminescence efficiency is not a simple function of the photoluminescence efficiency of the quantum dot films. even if the decreased charge-injection barrier/energy transfer distance of the shortest ligand may play a role, for conclusion that differences in film morphology resulting from the conflicting solubility of each type of quantum dot are more important in determining the performance of our devices.
Fig. 2. (a) Photoluminescence spectra of CdSe/CdZnS/ZnS quantum dots exchanged with octadecylamine (diamonds), octylamine (circles), and butylamine (triangles) in solutions of the same quantum dot awareness. (b) Solid state photoluminescence from the same quantum dots included into the QD-LED structure shown in Fig. 1
