Layer‐by‐Layer Assembly of 3D Tissue Constructs with Functionalized Graphene

Carbon‐based nanomaterials have been considered promising candidates to mimic certain structure and function of native extracellular matrix materials for tissue engineering. Significant progress has been made in fabricating carbon nanoparticle‐incorporated cell culture substrates, but only a limited number of studies have been reported on the development of 3D tissue constructs using these nanomaterials. Here, a novel approach to engineer 3D multilayer constructs using layer‐by‐layer (LbL) assembly of cells separated with self‐assembled graphene oxide (GO)‐based thin films is presented. The GO‐based structures are shown to serve as cell adhesive sheets that effectively facilitate the formation of multilayer cell constructs with interlayer connectivity. By controlling the amount of GO deposited in forming the thin films, the thickness of the multilayer tissue constructs could be tuned with high cell viability. Specifically, this approach could be useful for creating dense and tightly connected cardiac tissues through the co‐culture of cardiomyocytes and other cell types. In this work, the fabrication of stand‐alone multilayer cardiac tissues with strong spontaneous beating behavior and programmable pumping properties is demonstrated. Therefore, this LbL‐based cell construct fabrication approach, utilizing GO thin films formed directly on cell surfaces, has great potential in engineering 3D tissue structures with improved organization, electrophysiological function, and mechanical integrity.     

Layer‐by‐Layer Fabrication and Characterization of Gold‐Nanoparticle/Myoglobin Nanocomposite Films

Multilayer thin films of ～ 7 nm diameter gold nanoparticles (GNPs) linked with horse heart myoglobin (Mb) are fabricated, for the first time, by layer‐by‐layer (LbL) assembly on glass slides, and silicon and plastic substrates. The GNP/Mb nanocomposite films show sharp surface plasmon resonance (SPR) absorption bands that are used to follow the LbL growth of the film and to determine the kinetics of GNP adsorption on the Mb‐modified surface. The GNP/Mb nanocomposite films are characterized using atomic force microscopy, transmission electron microscopy, polarized UV‐vis spectroscopy, and spectroscopic ellipsometry. The GNPs in the multilayer films are spatially separated from one another, and interparticle interactions remain in the film, making it optically anisotropic. The GNP/Mb nanocomposite films are stable in air at temperatures up to 100 °C, and can withstand successive immersions in strongly acidic and basic solutions. The SPR absorption band of the GNP/Mb nanocomposite film in air exhibits a red‐shift in the wavelength maximum and an increase in the maximum absorbance relative to that in water. This result, which is in contrast to that observed with a GNP monolayer on an aminosilane‐functionalized substrate, suggests the shrinkage in air and swelling in water of Mb molecules embedded in the nanocomposite film.     

Flexible and Robust 2D Arrays of Silver Nanowires Encapsulated within Freestanding Layer‐by‐Layer Films

Freestanding layer‐by‐layer (LbL) films encapsulating controlled volume fractions (? = 2.5–22.5 %) of silver nanowires are fabricated. The silver nanowires are sandwiched between poly(allylamine hydrochloride)/poly(styrene sulfonate) (PAH/PSS) films resulting in nanocomposite structures with a general formula of (PAH/PSS)₁₀PAH Ag(PAH/PSS)₁₀PAH. The Young's modulus, toughness, ultimate stress, and ultimate strain are evaluated for supported and freestanding structures. Since the diameter of the nanowires (73 nm) is larger than the thickness of the LbL films (total of about 50 nm), a peculiar morphology is observed with the silver nanowires protruding from the planar LbL films. Nanowire‐containing LbL films possess the ability to sustain significant elastic deformations with the ultimate strain reaching 1.8 %. The Young's modulus increases with increasing nanowire content, reaching about 6 GPa for the highest volume fraction, due to the filler reinforcement effect commonly observed in composite materials. The ultimate strengths of these composites range from 60–80 MPa and their toughness reaches 1000 kJ m^–3 at intermediate nanowire content, which is comparable to LbL films reinforced with carbon nanotubes. These robust freestanding 2D arrays of silver nanowires with peculiar optical, mechanical, and conducting properties combined with excellent micromechanical stability could serve as active elements in microscopic acoustic, pressure, and photothermal sensors.     

Defect‐Tolerant Nanocomposites through Bio‐Inspired Stiffness Modulation

A biologically inspired, multilayer laminate structural design is deployed into nanocomposite films of graphene oxide‐poly(methyl methacrylate) (GO‐PMMA). The resulting multilayer GO‐PMMA films show greatly enhanced mechanical properties compared to pure‐graphene‐oxide films, with up to 100% increases in stiffness and strength when optimized. Notably, a new morphology is observed at fracture surfaces: whereas pure‐graphene‐oxide films show clean fracture surfaces consistent with crack initiation and propagation perpendicular to the applied tensile load, the GO‐PMMA multilayer laminates show terracing consistent with crack stopping and deflection mechanisms. As a consequence, these macroscopic GO‐PMMA films become defect‐tolerant and can maintain their tensile strengths as their sample volumes increase. Linear elastic fracture analysis supports these observations by showing that the stiffness modulation introduced by including PMMA layers within a graphene oxide film can act to shield or deflect cracks, thereby delaying failure and allowing the material to access more of its inherent strength. Together, these data clearly demonstrate that desirable defect‐tolerant traits of structural biomaterials can indeed be incorporated into graphene‐ oxide‐based nanocomposites.     

Layer‐by‐Layer Controlled Perovskite Nanocomposite Thin Films for Piezoelectric Nanogenerators

Perovskite nanoparticle‐based nanocomposite thin films strictly tailored using unconventional layer‐by‐layer (LbL) assembly in organic media for piezoelectric nanogenerators (NGs) are demonstrated. By employing sub‐20‐nm BaTiO₃ nanoparticles stabilized by oleic acid ligands (i.e., OA‐BTO_NPs) and carboxylic acid (COOH)‐functionalized polymers, such as poly(acrylic acid) (PAA), the resulting OA‐BTO_NP/PAA nanocomposite multilayers are prepared by exploiting the high affinity between the COOH groups of PAA and the BTO_NPs. The ferroelectric and piezoelectric performance of the (PAA/OA‐BTO_NP)_n thin films can be precisely controlled by altering the bilayer number, inserted polymer type, and OA‐BTO_NP size. It is found that the LbL assembly in nonpolar solvent media can effectively increase the quantity of adsorbed OA‐BTO_NPs, resulting in the dramatic enhancement of electric power output from the piezoelectric NGs. Furthermore, very low leakage currents are detected from the (PAA/OA‐BTO_NP)_n thin films for obtaining highly reliable power‐generating performance of piezoelectric NGs.     

Sub‐Micrometer Zeolite Films on Gold‐Coated Silicon Wafers with Single‐Crystal‐Like Dielectric Constant and Elastic Modulus

A low‐temperature synthesis coupled with mild activation produces zeolite films exhibiting low dielectric constant (low‐k) matching the theoretically predicted and experimentally measured values for single crystals. This synthesis and activation method allows for the fabrication of a device consisting of a b‐oriented film of the pure‐silica zeolite MFI (silicalite‐1) supported on a gold‐coated silicon wafer. The zeolite seeds are assembled by a manual assembly process and subjected to optimized secondary growth conditions that do not cause corrosion of the gold underlayer, while strongly promoting in‐plane growth. The traditional calcination process is replaced with a nonthermal photochemical activation to ensure preservation of an intact gold layer. The dielectric constant (k), obtained through measurement of electrical capacitance in a metal–insulator–metal configuration, highlights the ultralow k ≈ 1.7 of the synthetized films, which is among the lowest values reported for an MFI film. There is large improvement in elastic modulus of the film (E ≈ 54 GPa) over previous reports, potentially allowing for integration into silicon wafer processing technology.     

Covalent Layer‐by‐Layer Assembly of Conjugated Polymers and CdSe Nanoparticles: Multilayer Structure and Photovoltaic Properties

Hybrid thin films of conjugated polymers and CdSe nanoparticles have been fabricated by using a layer‐by‐layer (LbL) approach driven by covalent coupling reactions. This method permits facile covalent crosslinking of the polymer/nanoparticle interlayers in common organic solvents, which provides a general route for preparing robust and uniform functional thin films. The deposition process is linearly related to the number of bilayers as monitored by UV‐vis absorption spectroscopy and ellipsometry. Characterization of the multilayer structures has been carried out by fluorescence spectroscopy, X‐ray photoelectron spectroscopy (XPS), and grazing‐angle Fourier‐transform infrared spectroscopy (FTIR). Techniques such as atomic force microscopy (AFM) and scanning electron microscopy (SEM) have also been used. A preliminary application of the hybrid films in the development of organic photovoltaics is presented. Upon illumination with white light at 10 mW cm^–2, the self‐assembled multilayer films exhibit steady photocurrent responses with an overall optical‐to‐electrical power conversion efficiency of 0.71 %.     

Fabrication of a “Soft” Membrane Electrode Assembly Using Layer‐by‐Layer Technology

A new type of thin‐film electrode that does not utilize conducting polymers or traditional metal or chemical vapor deposition methods has been developed to create ultrathin flexible electrodes for fuel cells. Using the layer‐by‐layer (LbL) technique, carbon–polymer electrodes have been assembled from polyelectrolytes and stable carbon colloidal dispersions. Thin‐film LbL polyelectrolyte–carbon electrodes (LPCEs) have been successfully assembled atop both metallic and non‐metallic, porous and non‐porous substrates. These electrodes exhibit high electronic conductivities of 2–4 S cm^–1, and their porous structure provides ionic conductivities in the range of 10^–4 to 10^–3 S cm^–1. The electrodes show remarkable stability towards oxidizing, acidic, or delaminating basic solutions. In particular, an LPCE consisting of poly(diallyldimethyl ammonium chloride)/poly(2‐acrylamido‐2‐methyl‐1‐propane sulfonic acid)/carbon–platinum assembled on a porous stainless steel support yields an open‐circuit potential similar to that of a pure platinum electrode. With LbL carbon–polymer electrodes, the membrane‐electrode assembly (MEA) in a fuel cell can be made several times thinner, assume multiple geometries, and hence be more compact. The mechanism for LPCE deposition, electrode structure, and miniaturization will be presented and discussed, and demonstrations of the LbL electrodes in a traditional Nafion‐based proton fuel cell and the first demonstration of a thin‐film hydrogen–air “soft” fuel cell fully constructed using multilayer assembly are described.     

Inside Front Cover: Freely Suspended Layer‐by‐Layer Nanomembranes: Testing Micromechanical Properties (Adv. Funct. Mater. 5/2005)

A study of the micromechanical properties of layer‐by‐layer nanomembranes composed of a center layer of gold nanoparticles is reported by Tsukruk and co‐workers on p. 771. The micro‐ and nanomechanical properties of these membranes are measured using a combination of resonance‐frequency tests, bulging tests, and point‐load nanodeflection experiments. These freely suspended nanomembranes (right) with an elastic modulus of 5–10 GPa are very robust and can sustain multiple significant deformations (left, image obtained by B. Rybak and P. Kladitis). They are sensitive to variations in pressure and therefore have potential applications in pressure and acoustic sensors. Freely suspended nanocomposite layer‐by‐layer (LbL) nanomembranes composed of a central layer of gold nanoparticles sandwiched between polyelectrolyte multilayers are fabricated via spin‐assisted LbL assembly. The diameter of the circular membranes is varied from 150 to 600 μm and the thickness is kept within the range of 25–70 nm. The micro‐ and nanomechanical properties of these membranes are studied using a combination of resonance‐frequency and bulging tests, and point‐load nanodeflection experiments. Our results suggest that these freely suspended nanomembranes, with a Young's modulus of 5–10 GPa are very robust and can sustain multiple significant deformations. They are very sensitive to minor variations in pressure, surpassing ordinary semiconductor and metal membranes by three to four orders of magnitude and therefore have potential applications as pressure and acoustic microsensors.     

Freely Suspended Layer‐by‐Layer Nanomembranes: Testing Micromechanical Properties

Freely suspended nanocomposite layer‐by‐layer (LbL) nanomembranes composed of a central layer of gold nanoparticles sandwiched between polyelectrolyte multilayers are fabricated via spin‐assisted LbL assembly. The diameter of the circular membranes is varied from 150 to 600 μm and the thickness is kept within the range of 25–70 nm. The micro‐ and nanomechanical properties of these membranes are studied using a combination of resonance‐frequency and bulging tests, and point‐load nanodeflection experiments. Our results suggest that these freely suspended nanomembranes, with a Young's modulus of 5–10 GPa are very robust and can sustain multiple significant deformations. They are very sensitive to minor variations in pressure, surpassing ordinary semiconductor and metal membranes by three to four orders of magnitude and therefore have potential applications as pressure and acoustic microsensors.     

Ambient Layer‐by‐Layer ZnO Assembly for Highly Efficient Polymer Bulk Heterojunction Solar Cells

The use of metal oxide interlayers in polymer solar cells has great potential because metal oxides are abundant, thermally stable, and can be used in flexible devices. Here, a layer‐by‐layer (LbL) protocol is reported as a facile, room‐temperature, solution‐processed method to prepare electron transport layers from commercial ZnO nanoparticles and polyacrylic acid (PAA) with a controlled and tunable porous structure, which provides large interfacial contacts with the active layer. Applying the LbL approach to bulk heterojunction polymer solar cells with an optimized ZnO layer thickness of ≈25 nm yields solar cell power‐conversion efficiencies (PCEs) of ≈6%, exceeding the efficiency of amorphous ZnO interlayers formed by conventional sputtering methods. Interestingly, annealing the ZnO/PAA interlayers in nitrogen and air environments in the range of 60–300 °C reduces the device PCEs by almost 20% to 50%, indicating the importance of conformational changes inherent to the PAA polymer in the LbL‐deposited films to solar cell performance. This protocol suggests a new fabrication method for solution‐processed polymer solar cell devices that does not require postprocessing thermal annealing treatments and that is applicable to flexible devices printed on plastic substrates.     

Enhanced Electrochromism with Rapid Growth Layer‐by‐Layer Assembly of Polyelectrolyte Complexes

In this work, a facile method to deposit fast growing electrochromic multilayer films with enhanced electrochemical properties using layer‐by‐layer (LbL) self‐assembly of complex polyelectrolyte is demonstrated. Two linear polymers, poly(acrylic acid) (PAA) and polyethylenimine (PEI), are used to formulate stable complexes under specific pH to prepare polyaniline (PANI)/PAA‐PEI multilayer films via LbL deposition. By introducing polymeric complexes as building blocks, [PANI/PAA‐PEI]_n films grow much faster compared with [PANI/PAA]_n films, which are deposited under the same condition. Unlike the compact [PANI/PAA]_n films, [PANI/PAA‐PEI]_n films exhibit porous structure that is beneficial to the electrochemical process and leads to improved electrochromic properties. An enhanced optical modulation of 30% is achieved with [PANI/PAA‐PEI]₃₀ films at 630 nm compared with the lower optical modulation of 11% measured from [PANI/PAA]₃₀ films. The switching time of [PANI/PAA‐PEI]₃₀ films is only half of that of [PANI/PAA]₃₀ films, which indicates a faster redox process. Utilizing polyelectrolyte complexes as building blocks is a promising approach to prepare fast growing LbL films for high performance electrochemical device applications.     

Humidity‐Triggered Self‐Healing of Microporous Polyelectrolyte Multilayer Coatings for Hydrophobic Drug Delivery

Layer‐by‐layer (LbL) self‐assemblies have inherent potential as dynamic coatings because of the sensitivity of their building blocks to external stimuli. Here, humidity serves as a feasible trigger to activate the self‐healing of a microporous polyethylenimine/poly(acrylic acid) multilayer film. Microporous structures within the polyelectrolyte multilayer (PEM) film are created by acid treatment, followed by freeze‐drying to remove water. The self‐healing of these micropores can be triggered at 100% relative humidity, under which condition the mobility of the polyelectrolytes is activated. Based on this, a facile and versatile method is suggested for directly integrating hydrophobic drugs into PEM films for surface‐mediated drug delivery. The high porosity of microporous film enables the highest loading (≈303.5 μg cm^?2 for a 15‐bilayered film) of triclosan to be a one‐shot process via wicking action and subsequent solvent removal, thus dramatically streamlining the processes and reducing complexities compared to the existing LbL strategies. The self‐healing of a drug‐loaded microporous PEM film significantly reduces the diffusion coefficient of triclosan, which is favorable for the long‐term sustained release of the drug. The dynamic properties of this polymeric coating provide great potential for its use as a delivery platform for hydrophobic drugs in a wide variety of biomedical applications.     

Surface Imprinting in Layer‐by‐Layer Nanostructured Films

In this Full Paper, we develop a novel approach for the generation of stable molecularly imprinted sites in polymeric films by combining the layer‐by‐layer (LbL) technique and photochemical crosslinking of the layered structure. After photo‐crosslinking, the imprinted films show high reproducibility and rapid loading and unloading of imprinted sites by the template molecules. Moreover, the competitive adsorption of template molecules and redox labels into the imprinted film using electrochemical methods indicates that the imprinted film has higher affinity for template molecules. We believe this approach may have some advantages over traditional ways of preparing imprinted sites in polymer matrices and it may open a new avenue for the functionalization of LbL films.     

High‐Nanofiller‐Content Graphene Oxide–Polymer Nanocomposites via Vacuum‐Assisted Self‐Assembly

Highly ordered, homogeneous polymer nanocomposites of layered graphene oxide are prepared using a vacuum‐assisted self‐assembly (VASA) technique. In VASA, all components (nanofiller and polymer) are pre‐mixed prior to assembly under a flow, making it compatible with either hydrophilic poly(vinyl alcohol) (PVA) or hydrophobic poly(methyl methacrylate) (PMMA) for the preparation of composites with over 50 wt% filler. This process is complimentary to layer‐by‐layer assembly, where the assembling components are required to interact strongly (e.g., via Coulombic attraction). The nanosheets within the VASA‐assembled composites exhibit a high degree of order with tunable intersheet spacing, depending on the polymer content. Graphene oxide–PVA nanocomposites, prepared from water, exhibit greatly improved modulus values in comparison to films of either pure PVA or pure graphene oxide. Modulus values for graphene oxide–PMMA nanocomposites, prepared from dimethylformamide, are intermediate to those of the pure components. The differences in structure, modulus, and strength can be attributed to the gallery composition, specifically the hydrogen bonding ability of the intercalating species     

Spray Layer‐by‐Layer Electrospun Composite Proton Exchange Membranes

Polymer electrolyte films are deposited onto highly porous electrospun mats using layer‐by‐layer (LbL) processing to fabricate composite proton conducting membranes. By simply changing the assembly conditions for generation of the LbL film on the nanofiber mat substrate, three different and unique composite film morphologies can be achieved in which the electrospun mats provide mechanical support; the LbL assembly produces highly conductive films that coat the mats in a controlled fashion, separately providing the ionic conductivity and fuel blocking characteristics of the composite membrane. Coating an electrospun mat with the LbL dipping process produces composite membranes with “webbed” morphologies that link the fibers in‐plane and give the composite membrane in‐plane proton conductivities similar to that of the pristine LbL system. In contrast, coating an electrospun mat using the spray‐LbL process without vacuum produces a uniform film that bridges across all of the pores of the mat. These membranes have methanol permeability similar to free‐standing poly(diallyl dimethyl ammonium chloride)/sulfonated poly(2,6‐dimethyl 1,4‐phenylene oxide) (PDAC/sPPO) thin films. Coating an electrospun mat with the vacuum‐assisted spray‐LbL process produces composite membranes with conformally coated fibers throughout the bulk of the mat with nanometer control of the coating thickness on each fiber. The mechanical properties of the LbL‐coated mats display composite properties, exhibiting the strength of the glassy PDAC/sPPO films when dry and the properties of the underlying electrospun polyamide mat when hydrated. By combining the different spray‐LbL fabrication techniques with electrospun fiber supports and tuning the parameters, mechanically stable membranes with high selectivity can be produced, potentially for use in fuel cell applications.     

Porous Nanofilm Biomaterials Via Templated Layer‐by‐Layer Assembly

Hydrogel‐like biomaterials are often too soft to support robust cell adhesion, yet methods to increase mechanical rigidity (e.g., covalent cross‐linking the gel matrix) can compromise bioactivity by suppressing the accessibility or activity of embedded biomolecules. Nanoparticle templating is reported here as a strategy toward porous, layer‐by‐layer assembled, thin polyelectrolyte films of sufficient mechanical rigidity to promote strong initial cell adhesion, and that are capable of high bioactive species loading. Latex nanoparticles are incorporated during layer‐by‐layer assembly, and following 1‐ethyl‐3‐[3‐dimethylaminopropyl]carbodiimide/N‐hydroxysulfosuccinimide (EDC‐NHS) cross‐linking of the polyelectrolyte film, are removed via exposure to tetrahydrofuran (THF). THF exposure results in only a partial reduction in film thickness (as observed by ellipsometry), suggesting the presence of internal pore space. The attachment, spreading, and metabolic activity of pre‐osteoblastic MC3T3‐E1 cells cultured on templated, cross‐linked films are statistically similar to those on non‐templated films, and much greater than those on non‐cross‐linked films. Laser scanning confocal microscopy and quartz crystal microgravimetry indicate a high capacity for bioactive species loading (ca. 10% of film mass) in nanoparticle templated films. Porous nanofilm biomaterials, formed via layer‐by‐layer assembly with nanoparticle templating, promote robust cell adhesion and exhibit high bioactive species loading, and thus appear to be excellent candidates for cell‐contacting applications.     

Mechanical Properties of Robust Ultrathin Silk Fibroin Films

Robust ultrathin multilayer films of silk fibroin were fabricated by spin coating and spin‐assisted layer‐by‐layer assembly and their mechanical properties were studied both in tensile and compression modes for the first time. The ultrathin films were characterized by a high elastic modulus of 6–8 GPa (after treatment with methanol) with the ultimate tensile strength reaching 100 MPa. The superior toughness is also many times higher than that usually observed for conventional polymer composites (328 kJ m^–3 for the silk material studied here versus typical values of < 100 kJ m^–3). These outstanding properties are suggested to be caused by the gradual development of the self‐reinforcing microstructure of highly crystalline β‐sheets, serving as reinforcing fillers and physical crosslinks, a process that is well known for bulk silk materials but it is demonstrated here to occur in ultrathin films as well, despite their limited dimensions. However, the confined state within films thinner than the lengths of the extended domains causes a significantly reduced elasticity which should be considered in the design of nanosized films from silk materials. Such regenerated silk fibroin films with outstanding mechanical strength have potential applications in microscale biodevices, biocompatible implants, and synthetic coatings for artificial skin.     

A Delivery System for Self‐Healing Inorganic Films

Multilayer composites that utilize polymeric and brittle inorganic films are essential components for extending the lifetimes and exploiting the flexibility of many electronic devices. However, crack formation within the brittle inorganic layers that arise from defects as well as the flexing of these multilayer composite materials allows the influx of atmospheric water, a major source of device degradation. Thus, a composite material that can initiate self‐healing upon the influx of environmental water through defects or stress‐induced cracks would find potential applications in multilayer composite materials for permeation barriers. In the present study, the reactive metal oxide precursor TiCl₄ is encapsulated within the pores of a degradable polymer, poly(lactic acid) (PLA). Electrospun PLA fibers are found to be reactive to atmospheric water leading to the hydrolysis of the degradable polymer shell and subsequent release of the reactive metal oxide precursor. Release of the reactive TiCl₄ from the pores results in hydrolysis of the metal oxide precursor, forming solid titanium oxides at the surface of the fibers. The efficacy of this self‐healing delivery system is also demonstrated by the integration of these reactive fibers in the polymer planarization layer, poly(methyl methacrylate), of a multilayer film, upon which an alumina barrier layer is deposited. The introduction of nanocracks in the alumina barrier layer lead to the release of the metal oxide precursor from the pores of the fibers and the formation of titanium dioxide nanoparticles within the crack and upon the thin film surface. In this study the first delivery system that may find utility for the self‐healing of multilayer barrier films through the site‐specific delivery of metal oxide nanoparticles through smart reactive composite fibers is established.     

Energy‐Modulated Heterostructures Made with Conjugated Polymers for Directional Energy Transfer and Carrier Confinement

In this paper we demonstrate that multilayer structures with modulated bandgaps can be used for efficient energy transfer and carrier confinement inside a nanostructured film of a light‐emitting polymer. The films were produced with the layer‐by‐layer technique (LbL) with a poly(p‐phenylene vinylene) (PPV) precursor and a long chain dodecylbenzenesulfonate ion (DBS). DBS is incorporated selectively into the precursor chain, and with a rapid, low temperature conversion process (100 °C) superstructures with variable HOMO–LUMO gap could be formed along the deposition direction by changing the DBS concentration. Structures with different stair‐type energy modulations were produced, which are thermally stable and reproducible, as demonstrated by UV‐VIS. absorption measurements. Energy differences of up to 0.5 eV between the lowest and highest conjugated layers inside the stair structure could be achieved, which was sufficient to guide the excitation over long distances to the lower bandgap layer.