The growth and fluorescence of organic monolayers and heterostructures

Monolayer organic thin films and heterostructures are of great interest for their optical and electronic properties and as systems which allow the interplay between the structural and functional properties of organic molecules to be investigated. In the first experimental section of this thesis, sub-monolayer coverages of perylene tetracarboxylic diimide (PTCDI) were grown on hBN substrates and found to form needle-like monolayer islands at room temperature, while higher growth temperatures gave larger monolayer islands. The molecular packing of monolayer PTCDI was confirmed, using AFM, to correspond to the canted phase. The 0-0 fluorescence peak of this structure was found to occur at 2.208 ± 0.002 eV. The fluorescence of multi-layer PTCDI samples was mapped, with additional peaks measured at 2.135 ± 0.002 eV (580.7 ± 0.5 nm) and 2.118 ± 0.002 eV (585.4 ± 0.5 nm). Relating the morphology and fluorescence of such films using AFM and fluorescence microscopy is a promising way to investigate structural effects on the optical properties of multi-layer organic systems. Using solution deposition techniques, the PTCDI-melamine supramolecular network and the canted phase of PTCDI were deposited on hBN. The molecular packing of both structures was confirmed using AFM and the 0-0 fluorescence peaks were measured to be 2.245 ± 0.002 eV and 2.214 ± 0.002 eV for the PTCDI-melamine network and PTCDI respectively. The fluorescence of sublimed PTCDI, solution deposited PTCDI, PTCDI-melamine and measurements of Me-PTCDI doped helium nano droplets (HND) were compared. A 0.031 ± 0.002 eV red shift was measured from PTCDI-melamine to PTCDI while a 0.346 ± 0.002 eV red shift was measured from doped HND to PTCDI on hBN. A second perylene derivative, perylene tetracarboxylic dianhidride (PTCDA), was also deposited on hBN. Comparing the fluorescence of PTCDA monolayers on various dielectric substrates suggested a large shift due to the coupling of transition dipole moments and image dipoles beneath the dielectric surface. The shift between PTCDI and PTCDI-melamine was attributed to the coupling of transition dipole moments, for which the exciton bandstructure of both phases has been calculated with and without screening. The growth of sublimed C60 was also investigated, with monolayer islands observed for growth at room temperature and faceted bi-layer islands observed at 212 °C. The growth of PTCDI/C60 ¬heterostructures was also investigated, with C60 found to form monolayer islands on monolayer PTCDI at room temperature. At higher growth temperatures, C60 was found to form multilayers, with a reduced island density at PTCDI island edges, suggesting upward and downward hopping from the PTCDI surface to the second C60 layer and hBN respectively. C60 was found to quench the fluorescence of PTCDI and led to a 0.032 ± 0.02 eV blue shift. Finally, the growth of cyanuric acid-melamine (CA.M) on CVD graphene and CA.M/PTCDI heterostructures on hBN was investigated. Cyanuric acid-melamine was found to form monolayers with a honeycomb packing structure on CVD graphene. On monolayers of CA.M, PTCDI was found to form needle-like monolayer islands, the row direction of PTCDI is thought to have an on-axis registry with the substrate. Finally, the fluorescence of CA.M/PTCDI heterostructures on hBN was measured, with a 0.045 ± 0.002 eV blue shift from PTCDI on hBN

The growth and fluorescence of organic monolayers and heterostructures

Monolayer organic thin films and heterostructures are of great interest for their optical and electronic properties and as systems which allow the interplay between the structural and functional properties of organic molecules to be investigated. In the first experimental section of this thesis, sub-monolayer coverages of perylene tetracarboxylic diimide (PTCDI) were grown on hBN substrates and found to form needle-like monolayer islands at room temperature, while higher growth temperatures gave larger monolayer islands. The molecular packing of monolayer PTCDI was confirmed, using AFM, to correspond to the canted phase. The 0-0 fluorescence peak of this structure was found to occur at 2.208 ± 0.002 eV. The fluorescence of multi-layer PTCDI samples was mapped, with additional peaks measured at 2.135 ± 0.002 eV (580.7 ± 0.5 nm) and 2.118 ± 0.002 eV (585.4 ± 0.5 nm). Relating the morphology and fluorescence of such films using AFM and fluorescence microscopy is a promising way to investigate structural effects on the optical properties of multi-layer organic systems. Using solution deposition techniques, the PTCDI-melamine supramolecular network and the canted phase of PTCDI were deposited on hBN. The molecular packing of both structures was confirmed using AFM and the 0-0 fluorescence peaks were measured to be 2.245 ± 0.002 eV and 2.214 ± 0.002 eV for the PTCDI-melamine network and PTCDI respectively. The fluorescence of sublimed PTCDI, solution deposited PTCDI, PTCDI-melamine and measurements of Me-PTCDI doped helium nano droplets (HND) were compared. A 0.031 ± 0.002 eV red shift was measured from PTCDI-melamine to PTCDI while a 0.346 ± 0.002 eV red shift was measured from doped HND to PTCDI on hBN. A second perylene derivative, perylene tetracarboxylic dianhidride (PTCDA), was also deposited on hBN. Comparing the fluorescence of PTCDA monolayers on various dielectric substrates suggested a large shift due to the coupling of transition dipole moments and image dipoles beneath the dielectric surface. The shift between PTCDI and PTCDI-melamine was attributed to the coupling of transition dipole moments, for which the exciton bandstructure of both phases has been calculated with and without screening. The growth of sublimed C60 was also investigated, with monolayer islands observed for growth at room temperature and faceted bi-layer islands observed at 212 °C. The growth of PTCDI/C60 ¬heterostructures was also investigated, with C60 found to form monolayer islands on monolayer PTCDI at room temperature. At higher growth temperatures, C60 was found to form multilayers, with a reduced island density at PTCDI island edges, suggesting upward and downward hopping from the PTCDI surface to the second C60 layer and hBN respectively. C60 was found to quench the fluorescence of PTCDI and led to a 0.032 ± 0.02 eV blue shift. Finally, the growth of cyanuric acid-melamine (CA.M) on CVD graphene and CA.M/PTCDI heterostructures on hBN was investigated. Cyanuric acid-melamine was found to form monolayers with a honeycomb packing structure on CVD graphene. On monolayers of CA.M, PTCDI was found to form needle-like monolayer islands, the row direction of PTCDI is thought to have an on-axis registry with the substrate. Finally, the fluorescence of CA.M/PTCDI heterostructures on hBN was measured, with a 0.045 ± 0.002 eV blue shift from PTCDI on hBN

Detecting Trends in Abundance and Distribution of Seagrasses in Lake Worth Lagoon, Palm Beach County, Florida

Over the past 15 years, seagrass community stability has varied in estuaries throughout Florida. This study sought to model potential patterns of physiochemical parameters and community composition that may correlate with the fluctuation of seagrass populations in Lake Worth Lagoon (LWL), Palm Beach County, FL over time (2007–2019). Seven transects and 4 polygon areas throughout the LWL were established and stratified along a north—south gradient. Sites were sampled annually (May–August) for water quality, seagrass and macroalgal abundance, and community composition. Models developed to explain macrophyte abundance and composition were assessed using Akaike Information Criterion. Interaction between year and site best explained seagrass abundance and community composition in transect and polygon sites. Transect data revealed that seagrass and macroalgae declined after 2012 and continued until barely detected after 2016. This die off was not consistent for all transects and there was site variability in annual dissolved oxygen, nitrate, phosphate, and salinity levels. Polygon sites exhibited a shift in community composition after 2013, initially dominated by seagrass species Syringodium filiforme and Halodule wrightii before transitioning in 2012–2013 to Halophila decipiens and Caulerpa spp. Central lagoon sites transitioned to communities devoid of nearly all vegetative species. The loss of seagrass and the change in community composition could be explained by a transient dry period in 2012 and a subsequent inflow of freshwater. These events (sudden drought followed by an increase in freshwater) likely compounded the stress on the system between 2011–2014, leading to a drastic change in seagrass community

van der Waals-induced chromatic shifts in hydrogen-bonded two-dimensional porphyrin arrays on boron nitride

The fluorescence of a two-dimensional supramolecular network of 5,10,15,20-tetrakis(4-carboxylphenyl)porphyrin (TCPP) adsorbed on hexagonal boron nitride (hBN) is red shifted due to, primarily, adsorbate–substrate van der Waals interactions. TCPP is deposited from solution on hBN and forms faceted islands with typical dimensions of 100 nm and either square or hexagonal symmetry. The molecular arrangement is stabilized by in-plane hydrogen bonding as determined by a combination of molecular resolution atomic force microscopy performed under ambient conditions and density functional theory; a similar structure is observed on MoS2 and graphite. The fluorescence spectra of submonolayers of TCPP on hBN are red-shifted by ∼30 nm due to the distortion of the molecule arising from van der Waals interactions, in agreement with time-dependent density functional theory calculations. Fluorescence intensity variations are observed due to coherent partial reflections at the hBN interface, implying that such hybrid structures have potential in photonic applications

Triplet Excitation and Electroluminescence from a Supramolecular Monolayer Embedded in a Boron Nitride Tunnel Barrier

© 2019 American Chemical Society. We show that ordered monolayers of organic molecules stabilized by hydrogen bonding on the surface of exfoliated few-layer hexagonal boron nitride (hBN) flakes may be incorporated into van der Waals heterostructures with integral few-layer graphene contacts forming a molecular/two-dimensional hybrid tunneling diode. Electrons can tunnel through the hBN/molecular barrier under an applied voltage VSD, and we observe molecular electroluminescence from an excited singlet state with an emitted photon energy hν > eVSD, indicating upconversion by energies up to ∼1 eV. We show that tunneling electrons excite embedded molecules into singlet states in a two-step process via an intermediate triplet state through inelastic scattering and also observe direct emission from the triplet state. These heterostructures provide a solid-state device in which spin-triplet states, which cannot be generated by optical transitions, can be controllably excited and provide a new route to investigate the physics, chemistry, and quantum spin-based applications of triplet generation, emission, and molecular photon upconversion

Resonant tunnelling into the two-dimensional subbands of InSe layers

Two-dimensional (2D) van der Waals (vdW) crystals have attracted considerable interest for digital electronics beyond Si-based complementary metal oxide semiconductor technologies. Despite the transformative success of Si-based devices, there are limits to their miniaturization and functionalities. Here we realize a resonant tunnelling transistor (RTT) based on a 2D InSe layer sandwiched between two multi-layered graphene (MLG) electrodes. In the RTT the energy of the quantum-confined 2D subbands of InSe can be tuned by the thickness of the InSe layer. By applying a voltage across the two MLG electrodes, which serve as the source and drain electrodes to the InSe, the chemical potential in the source can be tuned in and out of resonance with a given 2D subband, leading to multiple regions of negative differential conductance that can be additionally tuned by electrostatic gating. This work demonstrates the potential of InSe and InSe-based RTTs for applications in quantum electronics.

Mobile phone applications and self-management of diabetes: a systematic review with meta-analysis, meta-regression of 21 randomized trials, and GRADE

We conducted a systematic review with meta‐analysis of randomized controlled trials that evaluated the effect of diabetes apps. 1550 participants from 21 studies were included. For type 1 diabetes, a significant 0.49% reduction in HbA1c was seen (95%CI 0.04 to 0.94; I2=84%), with unexplained heterogeneity and a low GRADE of evidence. For type 2 diabetes, using diabetes apps was associated with a mean reduction of 0.57% (95%CI 0.32 to 0.82, I2=77%). The results had severe heterogeneity that was explained by the frequency of HCP feedback. In studies with no HCP feedback, low frequency, and high frequency HCP feedback, the mean reduction is 0.24% (95%CI ‐0.02 to 0.49; I2=0%), 0.33% (95%CI 0.07 to 0.59; I2=47%), and 1.12% (95%CI 0.91 to 1.32; I2=0%) respectively, with high GRADE of evidence. There is evidence that diabetes apps improve glycemic control in type 1 diabetes patients. A reduction of 0.57% in HbA1c was found in type 2 diabetes patients. However, HCP functionality is important to achieve clinical effectiveness. Futures studies need to explore the cost‐effectiveness of diabetes apps and optimal intensity of HCP feedback

Fluorescence and Electroluminescence of J-Aggregated Polythiophene Monolayers on Hexagonal Boron Nitride

The photophysics of a semiconducting polymer is manipulated through molecular self-assembly on an insulating surface. Adsorption of polythiophene (PT) monolayers on hexagonal boron nitride (hBN) leads to a structurally induced planarization and a rebalancing of inter- and intrachain excitonic coupling. This conformational control results in a dominant 0–0 photoluminescence peak and a reduced Huang–Rhys factor, characteristic of J-type aggregates, and optical properties which are significantly different to both PT thin films and single polymer strands. Adsorption on hBN also provides a route to explore electroluminescence from PT monolayers though incorporation into hybrid van der Waals heterostructures whereby the polymer monolayer is embedded within a hBN tunnel diode. In these structures we observe up-converted singlet electroluminescence from the PT monolayer, with an excitation mechanism based upon inelastic electron scattering. We argue that surface adsorption provides a methodology for the study of fundamental optoelectronic properties of technologically relevant polymers

Electroluminescence from a phthalocyanine monolayer encapsulated in a van der Waals tunnel diode

Monolayers of free base phthalocyanine (H2Pc) are grown on monolayer and few-layer exfoliated flakes of hexagonal boron nitride (hBN) which are subsequently integrated into a van der Waals tunnel diode. This heterostructure consists of two thin hBN flakes between which the H2Pc monolayer is sandwiched and also incorporates upper and lower few-layer graphene contacts. When a voltage is applied between the contacts, a tunnel current flows and the embedded molecules can be excited resulting in the emission of photons with wavelengths which are close to the peaks observed in photoluminescence. We also observe electroluminescence at voltages where the energy gained by a tunnelling electron is lower than the energy of the emitted photon implying a multi-electron excitation pathway which we attribute to the formation of an intermediate triplet state. Our results provide insights into the differences in excitation and relaxation of molecules in supramolecular monolayers and bulk crystals and we discuss how the alignment of the energy levels of the molecules and contact layers determine the emission process