Langmuir-Blodgett Assembly Research Articles

Fabrication of hierarchical nanostructures using binary colloidal nanosphere assembly

In this paper, we investigate the self-assembly of hierarchical nanostructures using monodispersed nanospheres with two different diameters. Our approach is to use a two-step method where the assembly of larger 200 nm nanospheres is used to direct the assembly of smaller 50 nm particles. This self-assembly technique is based on Langmuir–Blodgett assembly and has low equipment cost when compared with traditional lithography methods. We examine the effects of substrate surface treatment, solution concentration ratio, and spin speeds on the quality of the hierarchical assembly. The fabricated samples are examined using optical and scanning electron microscopy to investigate assembly yield. Various defect types are identified and mitigated by process control. The ability to create more complex assembly can result in smaller features and can enhance the performance of photonics and nanostructured surfaces.

Electrochemical Fine-Tuning of the Chemoresponsiveness of Langmuir-Blodgett Graphene Oxide Films.

Graphene oxide has been widely deployed in electrical sensors for monitoring physical, chemical, and biological processes. The presence of abundant oxygen functional groups makes it an ideal substrate for integrating biological functional units to assemblies. However, the introduction of this type of defects on the surface of graphene has a deleterious effect on its electrical properties. Therefore, adjusting the surface chemistry of graphene oxide is of utmost relevance for addressing the immobilization of biomolecules, while preserving its electrochemical integrity. Herein, we describe the direct immobilization of glucose oxidase onto graphene oxide-based electrodes prepared by Langmuir-Blodgett assembly. Electrochemical reduction of graphene oxide allowed to control its surface chemistry and, by this, regulate the nature and density of binding sites for the enzyme and the overall responsiveness of the Langmuir-Blodgett biofilm. X-ray photoelectron spectroscopy, surface plasmon resonance, and electrochemical measurements were used to characterize the compositional and functional features of these biointerfaces. Covalent binding between amine groups on glucose oxidase and epoxy and carbonyl groups on the surface of graphene oxide was successfully used to build up stable and active enzymatic assemblies. This approach constitutes a simple, quick, and efficient route to locally address functional proteins at interfaces without the need for additives or complex modifiers to direct the adsorption process.

A novel strategy for fabrication of antibacterial Kirschner wire via Langmuir-Blodgett assembly

Kirschner wires (K-wires) are widely used as orthopedic external fixators due to their versatility and effectiveness. However, infection at the pin tract is a frequent and feared complication of K-wires. Thus, in this study, graphene oxide/copper ion (GO/Cu2+) complex was coated on K-wires surface via Langmuir-Blodgett (LB) assembly to endow K-wires with antibacterial and biocompatible properties. This new LB strategy enables the facile control of the coating microstructure, including the arrangement, density and number of layers to achieve adjustable GO/Cu2+ loading, sustained Cu2+ release behaviors. During the LB process, GO sheets floating on the water-air interface were oriented in an arrangement such that a significant number of edges were exposed. The GO/Cu2+ coating exhibited inherent antibacterial properties in vitro and in vivo through the sharp-edged sheet structure of GO, membrane stress and the generation of reactive oxygen species (ROS). Moreover, MC3T3-E1 cells on modified K-wires showed better adhesion, migration and proliferation abilities because of the synergistic effects of physical structure and chemical properties of the GO/Cu2+ coating. Thus, modified K-wires could be promising as potent antimicrobial materials for orthopedic surgery.

Microphase transitions of Langmuir-Blodgett lipid-assembled monolayers with new types of ceramides, ultra-long-chain ceramide and 1-O-acylceramide

HypothesisIntercellular lipid lamellae, consisting of ceramide, cholesterol, and free fatty acids, are the primary pathways for substances in the stratum corneum (SC). The microphase transition of lipid-assembled monolayers (LAMs), mimicking an initial layer of the SC, would be affected by new types of ceramides: ceramide with ultra-long chain (CULC) and 1-O-acylceramide (CENP) with three chains in different direction. ExperimentsThe LAMs were fabricated with varying the mixing ratio of CULC (or CENP) against base ceramide via Langmuir-Blodgett assembly. Surface pressure-area isotherms and elastic modulus-surface pressure plots were obtained to characterize π-dependent microphase transitions. The surface morphology of LAMs was observed by atomic force microscopy. FindingsThe CULCs favored lateral lipid packing, and the CENPs hindered the lateral lipid packing by lying alignment, which was due to their different molecular structures and conformations. The sporadic clusters and empty spaces in the LAMs with CULC were presumably due to the short-range interactions and self-entanglements of ultra-long alkyl chains following the freely jointed chain model, respectively, which was not noticeably observed in the neat LAM films and the LAM films with CENP. The addition of surfactants disrupted the lateral packing of lipids, thus weakening the LAM elasticity. These findings allowed us to understand the role of CULC and CENP in the lipid assemblies and microphase transition behaviors in an initial layer of SC.

Reliable fabrication of transparent conducting films by cascade centrifugation and Langmuir-Blodgett deposition of electrochemically exfoliated graphene.

Electrochemical exfoliation is an efficient and scalable method to obtain liquid-phase graphene. Graphene in solution, obtained through electrochemical exfoliation or other methods, is typically polydisperse, containing particles of various sizes, which is not optimal for applications. We employed cascade centrifugation to select specific particle sizes in solution and prepared thin films from those graphene particles using the Langmuir–Blodgett assembly. Employing centrifugation speeds of 3, 4, and 5 krpm, further diluting the solutions in different volumes of solvent, we reliably and consistently obtained films of tunable thickness. We show that there is a limit to how thin these films can be, which is imposed by the percolation threshold. The percolation threshold is quantitatively compared to results found in literature that are obtained using other, more complex graphene film fabrication methods, and is found to occur with a percolation exponent and percolative figure of merit that are of the same order as results in literature. A maximum optical transparency of 82.4% at a wavelength of 660 nm is obtained for these films, which is in agreement with earlier works on Langmuir–Blodgett assembled ultrasonic-assisted liquid-phase exfoliated graphene. Our work demonstrates that films that are in all respects on par with films of graphene obtained through other solution-based processes can be produced from inexpensive and widely available centrifugal post-processing of existing commercially available solutions of electrochemically exfoliated graphene. The demonstrated methodology will lower the entry barriers for new research and industrial uses, since it allows researchers with no exfoliation experience to make use of widely available graphene materials.

Graphene Oxide/Silver Nanoparticles Platforms for the Detection and Discrimination of Native and Fibrillar Lysozyme: A Combined QCM and SERS Approach.

We propose a sensing platform based on graphene oxide/silver nanoparticles arrays (GO/AgNPs) for the detection and discrimination of the native and toxic fibrillar forms of an amyloid-prone protein, lysozyme, by means of a combination of Quartz Crystal Microbalance (QCM) and Surface Enhanced Raman Scattering (SERS) measurements. The GO/AgNPs layer system was obtained by Langmuir-Blodgett assembly of the silver nanoparticles followed by controlled adsorption of GO sheets on the AgNPs array. The adsorption of native and fibrillar lysozyme was followed by means of QCM, the measurements provided the kinetics and the mechanism of adsorption as a function of protein concentration as well as the mass and thickness of the adsorbed protein on both nanoplatforms. The morphology of the protein layer was characterized by Confocal Laser Scanning Microscopy experiments on Thioflavine T-stained samples. SERS experiments performed on arrays of bare AgNPs and of GO coated AgNP after native, or fibrillar, lysozyme adsorption allowed for the discrimination of the native form and toxic fibrillar structure of lysozyme. Results from combined QCM/SERS studies indicate a general construction paradigm for an efficient sensing platform with high selectivity and low detection limit for native and amyloid lysozyme.

Ultraflat Langmuir–Blodgett assembled graphene oxide saturable-absorber films for pulsed near-infrared laser generation

Large-scale production of ultraflat broadband saturable-absorber films is highly desired for passive mode-locked solid-state lasers. However, the current vapour deposition and spin coating routes for fabricating saturable absorbers (SAs) are suffering from the limited flexibility in substrate choice and complexity of mass production processes. Here, we demonstrate an ultraflat carboxyl-functionalized graphene oxide (GO-COOH) SA film via Langmuir–Blodgett (LB) assembly for solid state laser mode-locking. Hydrophilic carboxyl groups from GO sheets weaken the aggregation effect thus contribute to the uniform and stable dispersion of GO sheets in water. Such GO suspensions are made into an ultrathin large-area graphene-based SA film by a LB assembly process ensuring high surface uniformity. The room-temperature and highly repeated operation for GO LB films avoids the thermal damage of GO sheets and improves the membrane repeatability. Consequently, the ultrathin GO-COOH SA shows the modulation depth (2.3%) and low saturation intensity (24.7 KW cm−2) under 1064 nm laser irradiation. By inserting the GO SA into a Nd:GdVO4 laser, both passive Q-switched (QS) and passive Q-switched mode-locked (QML) operations are also attained. The slope efficiency of QS laser is up to 35.6% and the maximum single pulse energy is 1.48 μJ. In particular, the QML pulses can be achieved stably and repeatedly with an average output power of 1.33 W and a pulse energy of 13.2 nJ. Our strategy provides a new concept for improving the modulation stability of graphene-based SAs and promoting their industrial application in pulsed solid-state lasers.

The interaction of graphene oxide-silver nanoparticles with trypsin: Insights from adsorption behaviors, conformational structure and enzymatic activity investigations

In this work, we synthesized graphene oxide-silver nanoparticles (GO-AgNPs) hybrids by one-pot method. Since there are relatively few reports on whether GO-AgNPs bind and change the structure and function of trypsin, A variety of methods were employed to systematically characterize the molecular interaction between GO-AgNPs and trypsin. Results exhibited that GO-AgNPs bound with trypsin to form a ground state complex. GO-AgNPs had higher adsorption capacity for trypsin compared with single GO. Langmuir-Blodgett assembly method was used to confirm that AgNPs did not interfere with the adsorption of trypsin by GO. The secondary structure and the microenvironment of amino acid residues of trypsin were altered after interacting with GO-AgNPs. In addition, GO-AgNPs can enhance the activity of trypsin and promote the hydrolysis of bovine serum protein (BSA) by trypsin. These findings provide important support for the application of GO-based nanocomposites in the efficient immobilization of enzymes.

Controlled fabrication of biocompatible graphene oxide Langmuir–Blodgett films by size and surface property manipulation

Controlled fabrication of uniform graphene oxide (GO) films is of great demand for many applications such as biomedical and biosensing materials. Surface-supported GO films with different surface coverage and density have been successfully fabricated by Langmuir–Blodgett (LB) method. The dialyzed GO sheets with sizes of smaller than 200 nm are used for LB assembly so as to achieve well control over the quality of the GO films. The dialysis procedure can remove soluble impurities from the GO sheets to adjust the surface composition and physicochemical properties. GO sheets within nanometer sizes can avoid edge folding and stacking during LB assembly and get better control of interactions between GO sheets. These aspects help to fabricate the uniform LB assemblies of GO sheets successfully, which can then be transferred onto the solid surface to give GO films with controlled surface coverage and density. The surface-supported GO films show high stability and biocompatibility, which can mediate cell growth and proliferation. The study not only provides ways for controlling the physicochemical properties of the GO sheets but also depicts the controlled fabrication of uniform GO films for specific applications.

Ultrafast humidity sensor based on liquid phase exfoliated graphene

Humidity sensing is important to a variety of technologies and industries, ranging from environmental and industrial monitoring to medical applications. Although humidity sensors abound, few available solutions are thin, transparent, compatible with large-area sensor production and flexible, and almost none are fast enough to perform human respiration monitoring through breath detection or real-time finger proximity monitoring via skin humidity sensing. This work describes chemiresistive graphene-based humidity sensors produced in few steps with facile liquid phase exfoliation followed by Langmuir–Blodgett assembly that enables active areas of practically any size. The graphene sensors provide a unique mix of performance parameters, exhibiting resistance changes up to 10% with varying humidity, linear performance over relative humidity (RH) levels between 8% and 95%, weak response to other constituents of air, flexibility, transparency of nearly 80%, and response times of 30 ms. The fast response to humidity is shown to be useful for respiration monitoring and real-time finger proximity detection, with potential applications in flexible touchless interactive panels.

Nanometer-Thick Films of Aligned ZnO Nanowires Sensitized with Au Nanoparticles for Few-ppb-Level Acetylene Detection

Wireless devices for environmental and health monitoring require the development of stable and highly sensitive thin metal oxide gas-sensing films. This paper reports one-step Langmuir–Blodgett assembly of ultrathin (20 nm) nanostructured films of aligned ZnO nanowires with 10 nm-wide v-grooves on the surface, followed by sensitization with Au nanoparticles (Au NPs) by sputtering and postannealing. The ridges of the nanopatterns are revealed as diffusion barriers capable of preventing Au NPs from sintering via particle migration and growth (PMC). The resulting ZnO Langmuir–Blodgett film decorated with high dispersion of small Au NPs functions as a highly responsive conductance switch, which demonstrates unprecedented sensitivity (37 ppm–1) and detection limit (3 ppb) toward acetylene (C2H2), a key marker gas for air pollution caused by anthropogenic emission. The fabrication method for the stabilized nanometric metal on the Langmuir–Blodgett film reported in this work not only provides a general strategy to improve the sensitivity of the thin sensing film but also can be applied to many metal nanodot-based applications, such as solar cells, plasmon-resonance waveguides, and biosensors.

Hierarchical Nanohybrids with Graphene Tubes-Networks Decorated Microspherical Graphene Assemblies for Supercapacitors

Graphene belongs to an emerging class of ultrathin carbon membrane materials with a high specific surface area, chemical stability, and high electrical and thermal conductivities. Owing to these intrinsic physicochemical characteristics, graphene has been extensively investigated for widespread applications in nanodevices, sensors, catalysis, energy storage systems, and biomedicine as an alternative to porous carbon. In particular, graphene has received increasing attention as an electrode material for electrical double-layer capacitors (EDLCs) due to its high specific surface area and high intrinsic electrical conductivity. Recently, there have been tremendous achievements for improving the gravimetric capacitance of graphene.However, due to the 2D nature of the graphene sheet, graphene can easily restack to form lamellar microstructures on the current collector during the electrode fabrication process. The restacking of the graphene sheets may greatly reduce the utilization of the electrode material and limit the electron and mass transport at the interface of electrode, which as a result leads to decreased capacitor performance.Taking these points into consideration, one strategy for preventing the restacking of graphene sheets is to use CNTs as nanospacers, given their high electrical pathways and large surface area. Recently, several approaches have been reported for fabricating graphene/CNT composites. The most popular approach for fabricating graphene/CNT composites is the preparation of an aqueous solution of GO and CNTs with the use of a surfactant such as sodium dodecylbenzenesulfonate (SDBS) to improve the dispersibility of CNTs, followed by vacuum filtration or a hydrothermal process. Another approach is the use of cationic surfactant polymers such as cetyltrimethylammonium bromide (CTAB) and polyethyleneimine (PEI) to induce electrostatic attraction by introducing a surface charge on CNTs or graphene. However, the use of insulating polymers is not desirable, since it is difficult to completely remove the surfactant polymers, and the amorphous carbon formed after heat treatment of the polymer under an inert atmosphere could deteriorate the electrical conductivity. Furthermore, such composites showed a marginal improvement of electrochemical properties.Another points, to further explore the macroscopic applications of graphene, an essential prerequisite is the controlled large-scale assembly of twodimensional (2D) graphene building blocks and the transfer of their inherent properties into three-dimensional (3D) structures with a high packing density. Furthermore, graphene needs to be assembled into a microsized powder form considering the powder morphology of activated carbon, which is currently used as an electrode material for EDLCs. Despite the recent progress in realizing graphene assemblies using well-established strategies such as vacuum filtration, layer-by-layer assembly, Langmuir–Blodgett assembly, hydrothermal assembly followed by oven drying, and spray drying, the controllable and scalable assembly of such graphene into graphene microsized powders with a high packing density remains a significant challengeIn this regards, herein, we report the novel integrated Graphene tube @ Graphene Microsphere by using cobalt acetate and dicyandiamide. Graphene tube @Graphene Microspheres (GT@GMs) show a high gravimetric capacitance of 229.8 F g-1 at 0.1 A g-1 in 1 M TEABF4/ACN electrolyte. The GT@GMs exhibited excellent rate capabilities of 94.3 % (gravimetric capacitance at a current density of 2 A g-1 compared with gravimetric capacitance at a current density of 0.1 A g-1) with time constants in the range of 0.4 to 0.8 s owing to the favorable formation of pathway by formation of graphene tubes; this allowed the electrolyte to easily penetrate the graphene assembly and facilitated ion transport. Furthermore, all the A-GMs exhibited excellent cycling stability (95.1 % of the initial capacitance retained after 100,000 charge/discharge cycles at a current density of 2 A g–1) owing to their low oxygen content as well as structural stability originating from the compact assembly of graphene micropores. This study provides new insights regarding the introducing of 1D carbon into graphene to produce graphene-based electrode materials with both high gravimetric capacitances and rate capabilities. Detailed synthetic procedure, electrochemical properties will be discussed at the meeting.

Continuous roll-to-roll patterning of three-dimensional periodic nanostructures

In this work, we introduce a roll-to-roll system that can continuously print three-dimensional (3D) periodic nanostructures over large areas. This approach is based on Langmuir-Blodgett assembly of colloidal nanospheres, which diffract normal incident light to create a complex intensity pattern for near-field nanolithography. The geometry of the 3D nanostructure is defined by the Talbot effect and can be precisely designed by tuning the ratio of the nanosphere diameter to the exposure wavelength. Using this system, we have demonstrated patterning of 3D photonic crystals with a 500 nm period on a 50 × 200 mm2 flexible substrate, with a system throughput of 3 mm/s. The patterning yield is quantitatively analyzed by an automated electron beam inspection method, demonstrating long-term repeatability of an up to 88% yield over a 4-month period. The inspection method can also be employed to examine pattern uniformity, achieving an average yield of up to 78.6% over full substrate areas. The proposed patterning method is highly versatile and scalable as a nanomanufacturing platform and can find application in nanophotonics, nanoarchitected materials, and multifunctional nanostructures.

Highly Ordered and Dense Thermally Conductive Graphitic Films from a Graphene Oxide/Reduced Graphene Oxide Mixture

Summary We report a new approach to making highly dense, oriented, and crystalline graphite films from heat-treated and pressed graphene oxide (G-O). By introducing small-diameter reduced graphene oxide (rG-O) flakes into the graphene oxide starting material, we found that after heat treatment at 3,000°C, the sample density and atomic order substantially improved over a film composed, at the outset, only of pure G-O flakes. A subsequent mechanical press increased the density but reduced the atomic order. A second 3,000°C heat treatment restored the graphitic structure with graphitization metrics exceeding even those of the first heat treatment. The optimized graphitic film with an original concentration of 15 wt % reduced G-O in G-O gave well-oriented graphitic films with a density of 2.1 g cm−3, cross-plane thermal conductivity of 5.65 W m−1 K−1, and in-plane thermal conductivity of 2,025 ± 25 W m−1 K−1.

Lead halide perovskite nanowires stabilized by block copolymers for Langmuir-Blodgett assembly

The rapid development of solar cells based on lead halide perovskites (LHPs) has prompted very active research activities in other closely-related fields. Colloidal nanostructures of such materials display superior optoelectronic properties. Especially, one-dimensional (1D) LHPs nanowires show anisotropic optical properties when they are highly oriented. However, the ionic nature makes them very sensitive to external environment, limiting their large scale practical applications. Here, we introduce an amphiphilic block copolymer, polystyrene-block-poly(4-vinylpyridine) (PS-P4VP), to chemically modify the surface of colloidal CsPbBr3 nanowires. The resulting core-shell nanowires show enhanced photoluminescent emission and good colloidal stability against water. Taking advantage of the stability enhancement, we further applied a modified Langmuir-Blodgett technique to assemble monolayers of highly aligned nanowires, and studied their anisotropic optical properties.

Electronic and optoelectronic applications of solution-processed two-dimensional materials

ABSTRACT The isolation of graphene in 2004 has initiated much interest in two-dimensional (2D) materials. With decades of development, solution processing of 2D materials has becoming very promising due to its large-scale production capability, and it is therefore necessary to examine progress on solution-processed 2D materials and their applications. In this review, we highlight recent advances in the assembly of solution-processed 2D materials into thin films and the use of them for electronics and optoelectronics. We first present an overview about typical approaches to assemble solution-processed 2D materials into desired structures, including layer-by-layer assembly, Langmuir–Blodgett assembly, spin coating, electrophoretic deposition, inkjet printing, and vacuum filtration. Then, electronic and optoelectronic applications of such assembly films are presented, such as thin-film transistors, transparent conductive films, mechanical and chemical sensors, photodetectors and optoelectronic devices, as well as flexible and printed electronics. Finally, our perspectives on challenges and future opportunities in this important field are proposed.

The thermal-to-electrical energy conversion in (Bi0.5Na0.5)0.94Ba0.06TiO3/graphene oxide heterogeneous structures

In this work, a pyroelectric device based on (Bi0.5Na0.5)0.94Ba0.06TiO3/graphene oxide (BNT-BT/GO) heterogeneous structures is designed and fabricated for the first time. BNT-BT/GO is obtained by a controlled Langmuir-Blodgett (L-B) assembly method, and the device for thermal-to-electrical energy conversion is fabricated from the BNT-BT/GO heterogeneous structures via an etching process. The morphology, pyroelectric properties and the thermal-to-electrical energy conversion of BNT-BT/GO have been characterized and evaluated. Notably, the fabricated BNT-BT/GO device exhibits a giant electrocaloric effect, and the refrigeration temperature change is ~7.09 K at 110 °C. More importantly, the heterogeneous structure is proved to have remarkable thermal-to-electrical energy conversion properties, whose energy density is as high as 1.361 J cm−3 with temperatures varying between 30 and 120 °C. The results demonstrate that the ferroelectric heterogeneous structures are promising for waste-heat harvesting.

Changing the Blood Test: Accurate Determination of Mercury(II) in One Microliter of Blood Using Oriented ZnO Nanobelt Array Film Solution-Gated Transistor Chips.

The measurement of ultralow concentrations of heavy metal ions (HMIs) in blood is challenging. A new strategy for the determination of mercury ions (Hg2+ ) based on an oriented ZnO nanobelt (ZnO-NB) film solution-gated field-effect transistor (FET) chip is adopted. The FET chips are fabricated with ZnO-NB film channels with different orientations utilizing the Langmuir-Blodgett (L-B) assembly technique. The combined simulation and I-V behavior results show that the nanodevice with ZnO-NBs parallel to the channel has exceptional performance. The sensing capability of the oriented ZnO-NB film FET chips corresponds to an ultralow minimum detectable level (MDL) of 100 × 10-12 m in deionized water due to the change in the electrical double layer (EDL) arising from the synergism of the field-induced effect and the specific binding of Hg2+ to the thiol groups (-SH) on the film surface. Moreover, the prepared FET chips present excellent selectivity toward Hg2+ , excellent repeatability, and a rapid response time (less than 1 s) for various Hg2+ concentrations. The sensing performance corresponds to a low MDL of 10 × 10-9 m in real samples of a drop of blood.

Self-Limiting Assembly Approaches for Nanoadditive Manufacturing of Electronic Thin Films and Devices.

Most electronics consist of functional thin films with tens of nanometer thicknesses. It is usually challenging to control the growth of these thin films using conventional solution-based approaches. Nanoadditive manufacturing, a method to deposit electronically desired molecules, polymers, or nanomaterials in a layer-by-layer (LbL) fashion, has emerged as a promising technique for the precise control of film growth and device fabrication. Here, basic principles of nanoadditive manufacturing approaches with self-limiting characteristics are summarized with a particular focus on Langmuir-Blodgett assembly and LbL assembly. Additively manufactured electronic thin films with properties of conductors, semiconductors, and dielectrics are reviewed, followed by a discussion of their application in various electronics, such as field-effect transistors, sensors, memory devices, photodetectors, light-emitting diodes, and electrochromic devices. Finally, challenges and future developments of these approaches are proposed. The resulting analysis reveals promising opportunities of nanoadditive manufacturing for the solution-based fabrication of electronic devices.

Langmuir-Blodgett self-assembly of ultrathin graphene quantum dot films with modulated optical properties.

Multiple-color-emissive graphene quantum dots (GQDs) have great potential in diverse applications such as bioimaging, light emission, and photocatalysis. Growing interest in GQDs is largely focused on their macro-scale aggregations that could inherit the unique properties of individual dots. However, the lack of advanced fabrication methods limits the practical applications of GQDs. Here, we employed a Langmuir-Blodgett (LB) technique to fabricate ultrathin, high-quality GQD aggregated films with well-modulated optical properties in a wide range of wavelengths. Through the combination of a bottom-up synthesis of GQDs and the LB assembly method, uniform, closely packed, and ultra-thin GQD films can be self-assembled with a well-controlled thickness on different substrates. The photoluminescence (PL) spectra of ultra-thin GQD films have an obvious red-shift compared with isolated GQD solution. We then elucidate remarkably strong energy transfer in self-assembled GQDs. Furthermore, the ultra-thin GQD films exhibit a clear excitation-dependent PL that could almost cover the entire visible light. This convenient self-assembly method and systematic optical and physical studies of ultra-thin GQD films may provide a new direction for developing low-cost, GQD film-based light-emitting devices.