Abstract:
A separated carbon nanotube-based active matrix organic light-emitting diode (AMOLED) device including a substrate and transistors. Each transistor includes an individual back gate patterned on the substrate and a gate dielectric layer disposed over the substrate. An active channel including a network of separated semiconducting nanotubes is disposed over a functionalized surface of the gate dielectric layer. A source contact and a drain contact are formed on two ends of the active channel, with the network of separated nanotubes between the source contact and the drain contact. An organic light-emitting diode (OLED) display device is coupled to the drain of one of the transistors. A system includes a display control circuit having a substrate, with scan lines, data lines, and AMOLED devices formed on the substrate, with each AMOLED device coupled to one of the scan lines and one of the data lines.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 U.S.C. §119(e) to U.S. Application Ser. No. 61/700,102 entitled “SEPARATED CARBON NANOTUBE-BASED ACTIVE MATRIX ORGANIC LIGHT-EMITTING DIODE DISPLAYS,” filed on Sep. 12, 2012, which is incorporated by reference herein in its entirety. 
     
    
     STATEMENT OF GOVERNMENT INTEREST 
       [0002]    This invention was made with government support under National Science Foundation Grant CCF-0702204 and under Defense Threat Reduction Agency Grant HDTRA1-10-1-0015. The government has certain rights in the invention. 
     
    
     TECHNICAL FIELD 
       [0003]    This invention is related to active matrix organic light-emitting diodes (AMOLEDs) having separated carbon nanotubes as well as displays including the same. 
       BACKGROUND 
       [0004]    Due at least in part to their high light efficiency, superior color purity, low power consumption, large view angle, excellent flexibility, and low temperature processing, organic light-emitting diodes (OLEDs) are promising candidates for the next generation display technologies. However, the fabrication of thin-film transistors (TFTs) in the active matrix (AM) backplane is still challenging. Unlike the requirement of driving transistors for traditional liquid crystal displays (LCDs), where amorphous silicon (a-Si) (mobility ˜1 cm 2 V −1 s −1 ) is applied as the transistor channel material, higher current driving capability is needed. Although polycrystalline silicon (poly-Si) which has better mobility (˜150 cm 2 V −1 s −1 ) is used as a temporary solution for AMOLED display transistors, its high cost, low transparency, high-temperature processing, short life time, and poor uniformity limits the commercial implementation of AMOLED displays. Other candidates such as organic semiconductor materials are also attractive, but similar to a-Si, they also suffer from low carrier mobilities. 
       SUMMARY 
       [0005]    Devices, systems, and methods are disclosed for implementing separated semiconducting nanotube-based active matrix organic light-emitting diode (AMOLED) displays. 
         [0006]    In a first general aspect, a device includes a substrate and transistors. Each of the transistors includes an individual back gate patterned on the substrate. A gate dielectric layer is disposed over the substrate, and a surface of the gate dielectric layer is functionalized with linker molecules. Each transistor includes an active channel. The active channel includes a network of separated nanotubes disposed over the functionalized surface of the gate dielectric layer. The network of separated nanotubes includes separated semiconducting nanotubes. Each transistor includes a source contact and a drain contact formed on two ends of the active channel, with the network of separated nanotubes therebetween. 
         [0007]    In a second general aspect, a system includes a display control circuit and an organic light-emitting diode (OLED) display device. The display control circuit includes a substrate and devices formed on the substrate. Scan lines and data lines are formed in the substrate. Each of the devices is coupled to a scan line and a data line. Each device includes transistors. Each of the transistors includes an individual back gate patterned on the substrate. A gate dielectric layer is disposed over the substrate, and a surface of the gate dielectric layer is functionalized with linker molecules. Each transistor includes an active channel including a network of separated nanotubes disposed over the functionalized surface of the gate dielectric layer. The network of separated nanotubes includes separated semiconducting nanotubes. Each of the transistors includes a source contact and a drain contact formed on two ends of the active channel with the network of separated nanotubes therebetween. The OLED display device includes OLED pixels, and each of the OLED pixels is coupled to a device of the display control circuit. 
         [0008]    Implementations can optionally include one or more of the following features. The gate dielectric layer can include a first dielectric layer disposed over the substrate and a second dielectric layer disposed over the first dielectric layer. The second dielectric layer can have better adhesion with the linker molecules than the first dielectric layer. The first and second dielectric layers can be 40-nm Al 2 O 3  and 5-nm SiO 2 , respectively. The substrate can include SiO 2  or silicon. The back gate dielectric material can include SiO 2 , Al 2 O 3 , or indium tin oxide (ITO). The surface of the gate dielectric layer can include SiO 2 , and the linker molecules can include amine groups. The surface of the gate dielectric layer can be functionalized by aminopropyltriethoxy silane (APTES) in isopropanol alcohol (IPA) solution with an APTES:IPA volume ratio of 1:10. The surface can be deposited with an enriched separated semiconducting nanotube solution with a concentration of 98%. 
         [0009]    In some cases, the active channel of each transistor has a length of 20 μm. The active channel can have a width of 100 μm. The active channel can have a density of 45 separated semiconducting nanotubes per μm 2 . In certain cases, an on/off ratio of the device exceeds 10 4 . 
         [0010]    The transistors include a first transistor and a second transistor. The gate of the first transistor is configured to receive a first voltage for controlling the first transistor. The source of the first transistor receives a signal. The drain of the first transistor is coupled to a gate of a second transistor. The source or the drain of the second transistor is configured to receive a second voltage. A current flowing across the second transistor can be associated with the signal and the second voltage. Each transistor can include a capacitor including first and second pins. The first pin can be coupled between the drain of the first transistor and the gate of the second transistor. The second pin can be coupled to the source of the second transistor. The capacitor can be configured to store and stabilize a voltage from the signal during a period. 
         [0011]    In some cases, the device includes an OLED display device coupled to the drain of the second transistor. An output light intensity of the OLED device can be modulated by the signal. A modulation of the output light intensity can exceed 10 5 . 
         [0012]    In certain cases, the OLED display device and the display control circuit are monolithically integrated on the substrate. The system can integrate, for example, 500 OLED pixels in the OLED display device and 1,000 transistors in the display control circuit for driving OLED pixels in the OLED display device. 
         [0013]    In another aspect, transistors are formed on a substrate, and a capacitor is coupled to the transistors. Forming each transistor on the substrate includes patterning an individual back gate on the substrate, disposing a gate dielectric layer over the substrate, functionalizing a surface of the gate dielectric layer with linker molecules, disposing an active channel including a network of separated nanotubes over the functionalized surface of the gate dielectric layer, and forming a source contact and a drain contact on two ends of the active channel with the network of separated nanotubes therebetween. The network of separated nanotubes includes separated semiconducting nanotubes. 
         [0014]    Implementations may include one or more of the following features. In one example, a SiO 2  layer is deposited on the gate dielectric layer to form a bilayer gate dielectric layer before functionalizing the surface of the gate dielectric layer. In some cases, the transistors include a first transistor and a second transistor, and coupling a capacitor to the transistors includes coupling a first pin of a capacitor between the drain of the first transistor and the gate of the second transistor, and coupling a second pin of the capacitor to the source contact of the second transistor. In certain cases, an organic light-emitting diode (OLED) display device is coupled to the drain contact of the second transistor. 
         [0015]    In some implementations, the transistors include a first transistor and a second transistor, the gate of the first transistor is configured to receive a first voltage for controlling the first transistor, the source of the first transistor is configured to receive a signal, the drain of the first transistor is coupled to a gate of the second transistor, the source or the drain of the second transistor is configured to receive a second voltage, a current flowing across the second transistor is associated with the signal and the second voltage, and the capacitor is configured to store and stabilize a voltage from the signal during a period. In some instances, an organic light-emitting diode display device is coupled to the drain of the second transistor, and an output light intensity of the organic light-emitting diode display device is modulated by the signal. 
         [0016]    The details of one or more disclosed implementations are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages will become apparent from the description, the drawings, and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIGS. 1A-1C  depict an AMOLED circuit and layout.  FIG. 1A  depicts a schematic diagram for an AMOLED circuit. Each pixel contains one switching transistor, one driving transistor, one charge storage capacitor, and an OLED.  FIG. 1B  depicts a top view for the layout of a single pixel AMOLED with an area of 500×500 μm 2 .  FIG. 1C  is a cross-sectional view for an AMOLED pixel including a glass substrate, patterned Ti/Au gate electrode, Al 2 O 3  gate dielectric, separated carbon nanotube (CNT) thin film for the active channel, Ti/Pd source and drain contacts, integrated OLED (indium tin oxide (ITO)/N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (NPD)/tris(8-hydroxyquinolinato)aluminum (Alq 3 )/LiF/Al), and a SiO 2  passivation layer. 
           [0018]      FIGS. 2A-2C  show field emission scanning electron microscopy (FE-SEM) images of separated CNT thin-films with different densities obtained by tuning the volume ratio of APTES and IPA used in the surface functionalization process.  FIG. 2D  shows a relationship between nanotube film density and APTES:IPA volume ratio.  FIGS. 2E and 2F  show channel length dependence of device on/off ratio and normalized on-current for transistors fabricated on nanotube films, respectively, using different APTES:IPA volume ratios. 
           [0019]      FIG. 3A  is a schematic diagram of a back-gated transistor built on separated nanotube thin-film with Ti/Au (5 Å/40 nm) back gate, Ti/Pd (5 Å/50 nm) contact electrodes, and Al 2 O 3 /SiO 2  bilayer gate dielectric (40 nm/5 nm).  FIG. 3B  is a FE-SEM image showing the channel of a back-gated SN-TFT with 20 μm channel length.  FIGS. 3C and 3E  show scanning electron microscope (SEM) images of separated nanotube thin-films deposited on Al 2 O 3  and Al 2 O 3 /SiO 2  surface, respectively.  FIGS. 3D and 3F  show SEM images for the samples shown in  FIGS. 3C and 3E , respectively, after one step of photolithography.  FIG. 3G  shows transfer (I D -V G ) characteristics and g m -V G  characteristics of a typical SN-TFT (L=20 μm, W=100 μm) with V D =1 V.  FIG. 3H  shows output (I D -V D ) characteristics of the same device with V G  varying from −5 V to 5 V in 1 V steps as indicated by the arrow. 
           [0020]      FIGS. 4A-4D  are related to two-transistor single-pixel circuits.  FIG. 4A  shows an optical microscope image of the single pixel circuit with two SN-TFTs, one capacitor, and the ITO electrode for OLED integration.  FIGS. 4B and 4C  show transfer (I D -V DATA ) characteristics of the single pixel circuit measured while V SCAN =−5 V in linear scale (V DD =1 V to 0.2 V with 0.2 V steps in the direction indicated by the arrow) and logarithm scale (V DD =1 V), respectively. The inset in  FIG. 4C  shows a schematic diagram of the single pixel circuit.  FIG. 4D  shows output (I D -V DD ) characteristics measured at V SCAN =−5 V with different V DATA  (−5 V to 5 V with 1V steps in the direction indicated by the arrow). 
           [0021]      FIG. 5  shows the current through an OLED (I OLED ) and OLED light intensity versus the voltage applied across the OLED (V OLED ) for a two-terminal measurement. 
           [0022]      FIG. 6A  shows characteristics of the OLED controlled by single pixel circuit, where the current flow through the OLED (I OLED ) is measured by sweeping the V DD . The curves correspond to values of V DATA  from −5 to 5 V in 1 V steps in the direction indicated by the arrow.  FIG. 6B  shows plots of current through the OLED (I OLED ) and OLED light intensity versus V DATA  with V DD =8 V. 
           [0023]      FIG. 7A  shows an image of an AMOLED substrate containing 7 AMOLED elements, each with 20×25 pixels.  FIG. 7B  is a photograph showing the pixels on an integrated AMOLED are turned on when V DATA =−5 V, V SCAN =−5V, and V DD =8 V are applied for the pixels. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Compared with other channel materials, one-dimensional nanoscale materials such as semiconductor nanowires (NWs) and single-walled carbon nanotubes (SWNTs) have advantages in terms of mobility, transparency, flexibility, and low temperature processing. AMOLED displays using NWs as the active channel materials. The device uniformity, reliability, and processing scalability can be improved by using pre-separated semiconducting nanotubes produced by density-gradient ultracentrifuge separation methods to yield transistors that exhibit highly uniform electrical performance. The use of high purity semiconducting nanotubes (referred to herein as “separated nanotubes,” “separated carbon nanotubes,” “separated semiconducting nanotubes,” and the like) allows a high on/off ratio (&gt;10 5 ), as well as excellent on-current density (˜1 μA/μm), which makes such separated carbon nanotube thin-film transistors (SN-TFTs) very attractive for AMOLED display applications. 
         [0025]    AMOLED displays driven by SN-TFTs demonstrate high light efficiency, flexibility, lightweight, and low-temperature processing. The high mobility, high percentage of semiconducting nanotubes, and room-temperature processing compatibility of these SN-TFTs allow large-scale high-yield fabrication of devices with superior performance, carbon nanotube film density optimization, bilayer gate dielectric for improved substrate adhesion to the deposited nanotube film, and monolithically integrated AMOLED display elements with 500 pixels driven by 1,000 SN-TFTs. AMOLED displays described herein can be used in nanotube-based thin-film display electronics. 
         [0026]    A monolithically integrated AMOLED display with SN-TFT based control circuit is described, and carbon nanotube film density is optimized with respect to transistor electrical performance. In addition, the single pixel control circuits including two SN-TFTs and one capacitor are made, and their OLED control capability is demonstrated. AMOLED display elements with 20×25 pixels driven by 1,000 SN-TFTs are fabricated and tested. Compared with conventional platforms, the SN-TFT platform shows advantages such as low temperature processing compatibility, scalability, reproducibility and device performance, and provides a practical and realistic approach for carbon nanotube based AMOLED display applications. 
         [0027]      FIG. 1A  depicts a schematic diagram of AMOLED circuit  100  with pixels  102 . Each pixel  102  contains one switching transistor (Ts)  104 , one driving transistor (Td)  106 , one charge storage capacitor (Cs)  108 , and one OLED  110 . The switching transistor  104 , controlled by a signal from a scan line  112 , is employed to select one specific row of pixels in an AMOLED display element by passing the signal from a data line  114  through the channel of the switching transistor  104  to the gate of the driving transistor  106 . The driving transistor  106  further controls the output light intensity of the OLED pixel  110  by modulating the current flowing through OLED. The capacitor Cs  108  is used to store and stabilize the voltage obtained from the data line  114  during one scanning period for line-by-line scanning technique typically used for dynamic displays. 
         [0028]    Based on the circuit diagram, the corresponding layout of one pixel is shown in  FIG. 1B  (top view) and  FIG. 1C  (cross-sectional view). The single pixel layout has a total area of 500×500 μm 2  with OLED area of 200×200 μm 2 , and is designed to be fabricated on glass substrate  120  with patterned Ti/Au (5 Å/40 nm) gate electrode  122 , Al 2 O 3  (40 nm) gate dielectric  124 , separated nanotube thin-film for the active channel  126 , Ti/Pd (5 Å/50 nm) source contacts and drain contacts  128 , integrated green OLED  130 , and a 200 nm SiO 2  passivation layer  132 . The total fabrication consists of 7 photo masks and 15 fabrication steps. 
         [0029]    To control the OLED intensity, the transistors in the control circuits have high current on/off ratio and excellent current drive capability. Shorter channel length and higher nanotube channel network density are understood to lead to high on-current density, which is needed for OLED display applications. However, it will also create more metallic nanotube pass in the channel, which may negatively affect the transistor current on/off ratio. Therefore, optimized device geometry and channel nanotube network density is understood to be a factor in OLED control. 
         [0030]    98% semiconducting carbon nanotube solution (from Nanointegris, Inc. Batch No. S08-665) is used, and uniform separated nanotube thin-film is achieved on a Si/SiO 2  surface by a solution-based aminopropyltriethoxy silane (APTES)-assisted separated nanotube deposition technique known in the art. The nanotube network density can be controlled by tuning the concentration of APTES in isopropanol alcohol (IPA) used for SiO 2  surface treatment before nanotube deposition. Three different conditions are studied (APTES:IPA=1:1, 1:10, 1:100), and the FE-SEM images of the resulting nanotube thin-film are shown in  FIGS. 2A-2C , respectively. From the images, the nanotube density is found to vary when different volume ratios of APTES and IPA are used. From  FIG. 2D , the sample with an APTES:IPA ratio of 1:1 has low nanotube network density (4 tubes/μm 2 ) and the uniformity of the thin-film is poor. For the sample with an APTES:IPA ratio of 1:10, a highly uniform film is obtained with a density of 45 tubes/μm 2 . If the solution is further diluted to APTES:IPA=1:100, the resulting film density decreases to 36 tubes/μm 2 . 
         [0031]    The relationship between nanotube film density and APTES:IPA ratio can be understood as follows: the effect of APTES is to functionalize the SiO 2  surface and form an amine-terminated monolayer, which can attract the nanotubes in solution to the substrate and form a uniform thin-film. When the APTES concentration is very high, instead of a uniform monolayer, multiple layers of APTES molecules are stacked onto the SiO 2  surface, leading to an uneven, low density nanotube film. As the APTES concentration in IPA is diluted, uniform monolayer APTES molecules are formed, which yields a highly uniform nanotube film with excellent density. However, when the APTES solution is diluted even further, the APTES monolayer may have defects and vacancies, so the nanotube film density may decrease again. Overall, by tuning the concentration of APTES in IPA, separated nanotube thin-film with different densities can be achieved. 
         [0032]    Subsequently, electrical performance of the nanotube network with different density was investigated. 100 transistors with different channel geometry were fabricated on each sample with different nanotube density, and the channel length dependence of device on/off ratio and normalized on-current are shown in  FIGS. 2E and 2F , respectively. From these two plots, it is seen that, with the benefit of high purity semiconducting nanotubes, all the devices with channel lengths larger than 20 μm have on/off ratios higher than 10 4 , and transistors made with nanotube film deposited using an APTES:IPA volume ratio of 1:10, which gives the highest nanotube density, also offer good current driving capability (0.5 μA/μm for 20 μm channel length devices). Based on the electrical performance, APTES:IPA volume ratio of 1:10 and device geometry of L=20 μm, W=100 μm are chosen as optimized conditions for transistors used in AMOLED control circuits. 
         [0033]      FIGS. 3A-3H  show structure and electrical characteristics of the nanotube transistors used in the AMOLED. For the sake of a two transistor control circuit, individual back-gated device structure  300  is chosen as illustrated in  FIG. 3A . 5 Å Ti and 40 nm Au are patterned on substrate  302  as the back gate  304 , and 40 nm Al 2 O 3  is deposited by atomic layer deposition (ALD) as the gate dielectric  306 . In some examples, due at least in part to poor adhesion between Al 2 O 3  and APTES molecules, the deposited nanotube thin-film on Al 2 O 3  surface can peel off during the ensuing fabrication steps. 
         [0034]    To improve the adhesion, a thin layer of SiO 2  (5 nm)  308  can be deposited on top of the Al 2 O 3  layer  306  using an electron beam evaporator to form a bilayer gate dielectric. With the help of the SiO 2  buffer layer, a uniform nanotube thin film  310  is achieved, as shown in  FIG. 3B . After the separated nanotube thin-film deposition, Ti/Pd (5 Å/50 nm) is applied on top of the channel network to form ohmic source and drain contacts  312  and  314 . Subsequently, the nanotubes outside the channel region can be etched away by photolithography and oxygen plasma. 
         [0035]      FIGS. 3C and 3E  show SEM images of the separated nanotube thin-films deposited on Al 2 O 3  and Al 2 O 3 /SiO 2  surfaces, respectively.  FIGS. 3D and 3F  show SEM images corresponding to the samples shown in  FIGS. 3C and 3E , respectively, after one step of photolithography.  FIG. 3D  shows that nanotubes on the Al 2 O 3  sample peeled off while, while  FIG. 3F  show that nanotubes on the Al 2 O 3 /SiO 2  bilayer dielectric still stick to the surface. 
         [0036]    Electrical properties of a typical transistor are plotted in  FIG. 3G , which contains transfer (I D -V G ) characteristics (plot  320  for linear scale and plot  322  for log scale) and g m -V G  characteristics (plot  324 ) measured with V D =1 V. The on-current at V D =1 V and V G =−5 V is 82.9 μA, corresponding to a current density of 0.829 μA/μm. The on/off ratio exceeds 10 4  and the peak transconductance is 25.5 μS. Based at least in part on the transconductance, the device mobility is extracted to be 31.65 cm 2 V −1 s −1 . A parallel plate model is used to estimate the gate capacitance when calculating the device mobility due to the complexity of the bilayer gate dielectric structure. If the electrostatic coupling between nanotubes is taken into consideration, the gate capacitance will be smaller and therefore the real mobility can be larger than the value listed here.  FIG. 3H  shows the output (I D -V D ) characteristics of the same device measured with V G  varying from −5 V to 5 V in 1 V steps, which indicates nice field-effect operation and ohmic contacts. 
         [0037]    Following the single transistor analysis, the AMOLED pixel control circuits were fabricated and studied.  FIG. 4A  shows an optical microscope image of the fabricated single pixel circuit  400  before OLED, which contains two SN-TFTs as a switching transistor  402  and a driving transistor  404 , one capacitor  406 , and one indium-tin oxide (ITO) electrode  408  for OLED integration. 
         [0038]    To operate the driving transistor, −5 V was applied to a scan line to turn on the switching transistor. Transfer (I D -V DATA ) characteristics and are plotted in  FIGS. 4B and 4C  in linear scale and logarithm scale, respectively. The various curves in  FIG. 4B  correspond to various values of the supply voltage V DD  (0.2 V to 1 V with 0.2 V steps), which is connected to the source of the driving transistor as shown in the inset schematic diagram in  FIG. 4C . From the transfer characteristics in logarithm scale, the two-transistor circuits are seen to exhibit an excellent on/off ratio (higher than 10 6 ), which resulted at least part from the optimized channel geometry and film density as well as the high semiconducting nanotube purity. This on/off ratio is crucial in order to guarantee that the control circuits can fully turn off the OLED pixels. 
         [0039]    Besides the on/off ratio, the current-drive of the circuit is also important for AMOLED displays, which is examined by the output (I D -V DD ) characteristics shown in  FIG. 4D . To keep the V GS  value of the driving transistor constant, the source of the driving transistor is grounded while the drain terminal (−V DD ) is swept from 0 V to −7 V, and different curves are obtained with V DATA  changing from −5 V to 5 V with 1 V steps. From this figure, current flow through the driving transistor is seen to saturate under high V DD , and with the optimized semiconducting nanotube density, 50 μA is achieved when V DD =3 V, V DATA =−5 V, and V SCAN =−5 V, which offers high enough current density to drive OLED pixels with a designed area of 200×200 μm 2 . 
         [0040]    To further understand the behavior of the circuit controlled AMOLED, an OLED was connected to and controlled by a single pixel control circuit using wire bonding. Standard NPD/Alq 3  OLED with multi-layered configuration was employed, with ITO/4-4′-bis[N-(1-naphthyl)-N-phenyl-amino]bi-phenyl (NPD) [40 nm]/tris(8-hydroxyquinoline) aluminium (Alq 3 ) [40 nm]/LiF [1 nm]/aluminum (Al) [100 nm], whose transfer characteristics are described herein with respect to  FIG. 5 .  FIG. 5  shows two terminal measurement of the OLED showing the current through the OLED (I OLED ) (plot  500 ) and OLED light intensity (plot  502 ) versus the voltage applied across the OLED (V OLED ). 
         [0041]    The schematic of the OLED control circuit is shown in the inset of  FIG. 6A , where the drain of the driving transistor is connected to an external OLED and a negative voltage (−V DD ) is applied to the cathode of the OLED. The current flow through the OLED (I OLED ) is measured by sweeping the V DD  while also changing input voltage V DATA  as plotted in  FIG. 6A . V SCAN  is held at −5 V to keep the switching transistor on, and the family of curves correspond to various values of V DATA  from −5 to 5 V in 1 V steps.  FIG. 6A  illustrates that if V DATA  is sufficiently negative, the OLED will be turned on when the supply voltage is higher than the threshold voltage of the OLED (about 3 V), and the current flow through OLED will increase as V DATA  decreases. Therefore, the light intensity of the OLED can be modulated by V DATA , which is directly revealed in  FIG. 6B  where current and output light intensity versus V DATA  characteristics (plots  600  and  60 ) with a fixed V DD  of 8 V as shown in the inset schematic. From  FIG. 6B , it can be seen that when sweeping V DATA  from −5 V to 5 V, the current through OLED changes from 71 μA to 3.7 nA and the output light intensity also varies from 5.3×10 −6  W/cm 2  to about 8×10 −12  W/cm 2 , which exceeds 5 orders of magnitude difference. The significant change in the light intensity can be visually seen when the OLED is under various V DATA  voltages of −5, −3, −1, 1, 3, and 5 V, respectively, demonstrating that the external OLED can be fully turned on and turned off by changing the voltage of V DATA . 
         [0042]    Based on the discussion herein, a monolithically integrated AMOLED display element was fabricated. First, an array of AMOLED control circuit with 20×25 pixels driven by 1,000 SN-TFTs was fabricated using the same layout design as discussed herein. After that, 200 nm SiO 2  was deposited by electron beam evaporator as a passivation layer, leaving only the pre-patterned ITO electrodes open for OLED integration. Finally, green OLEDs with the same multilayer structure and thickness (ITO/NPD/Alq 3 /LiF/Al) as used for the single pixel circuit were deposited by thermal evaporator onto ITO electrodes. 
         [0043]    An optical image of a completed AMOLED substrate, which contains 7 AMOLED elements (20×25 pixels each) is shown in  FIG. 7A .  FIG. 7B  is a photograph showing that all the pixels on one integrated AMOLED element are turned on when V DATA =−5 V, V SCAN =−5V, and V DD =8 V are applied for all the pixels. In  FIG. 7B , 348 out of 500 pixels are turned on, corresponding to a yield of 70%, which is acceptable for the demonstration purpose in the present laboratory-scale experiments. In some cases, the failed pixels are due to the top SiO 2  surface roughness, which can lead to short circuits during OLED evaporation, and can be improved by using an alternative passivation technique. This AMOLED display is understood to be driven solely by SN-TFT circuits. 
         [0044]    In summary, by tuning the concentration of APTES in IPA solution during surface functionalization step, an optimized separated nanotube thin-film density of 45 tubes/μm 2  is achieved when 1:10 volume ratio between APTES and IPA is used. Based on the optimized nanotube density and device geometry, individual back-gated transistors with superior on/off ratio (&gt;10 4 ) and excellent current driving capability (˜0.8 μA/μm) have been fabricated with 10 μm channel length and 100 μm channel width. In addition, the electrical properties and OLED control capability of the single pixel AMOLED control circuit were examined and analyzed, and the modulation in the output light intensity was shown to exceed 10 5 . Moreover, a monolithically integrated AMOLED display element with 500 OLED pixels and 1,000 transistors was further demonstrated. 
         [0045]    Further modifications and alternative embodiments of various aspects will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only. It is to be understood that the forms shown and described herein are to be taken as examples of embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description.