Organic Layer Thickness Research Articles

Vegetation Succession on Degraded Sites in the Pomacochas Basin (Amazonas, N Peru)—Ecological Options for Forest Restoration

The Andes of northern Peru are still widely covered with forests, but increasingly suffer from habitat fragmentation. Subsequent soil degradation often leads to the abandonment of overused forests and pastures. Ecological knowledge on the restoration potential, e.g., on dependencies of soil conditions and altitude, is scarce. Therefore, we compared soil and vegetation patterns along nine transects within the upper Pomacochas Basin, which is an important biodiversity corridor along the Andes, between remaining forests, succession sites and pastures. Anthropogenic successional and disturbance levels, geological substrate, and altitude have the most important ecological impacts on vegetation and tree species composition. Species responded to sandstone versus calcareous substrates, but also to depths of the organic soil layer, and light conditions. The absence of organic layers under pastures contrasted with the accumulation of thick organic layers under forest cover. Vegetation composition at succession sites revealed certain starting points (herbal stage, bush stage, or secondary forest) for restoration that will determine the length of regeneration paths. Pre-forest patches of Alchornea sp. and Parathesis sp. may act as habitat stepping stones for expeditiously restoring biocorridors for wildlife. The key findings can contribute to the sustainable use and conservation of biodiversity in a fragile ecoregion.

Latitudinal transect relationship between soil organic horizons and permafrost depth in Alaska

In this contribution, we examined a latitudinal soil transect from interior Alaska, focusing on morphological, physical and chemical characteristics of permafrost affected soils. The aim of this work was to evaluate if latitude (a proxy for temperature change) or other soil forming factors may affect organic layer thickness and, consequently, permafrost depth along the latitudinal gradient of 65° to 69°. Along this gradient, the plant community changed from boreal forest to moist tundra, but this change did not affect the amount of C stored in the organic horizons and the permafrost depth. Hence, we suspected that, at latitudes higher than the 65° degree, the formation of thick and thermally insulating organic layers is controlled mostly by the extent of the pedoturbations produced by the permafrost dynamics and the soil parent material.

Tangel humus forms − genesis and co-evolution with vegetation

Tangel humus consists of thick zoogenic organic layers up to 100cm which form directly over solid or coarse carbonate bedrock. Mineral soil horizons are absent or negligible. It develops mainly in the montane and subalpine region of the Northern Limestone Alps, where cool and moist climate, as well as slowly decomposable litter from coniferous trees and dwarf shrubs promotes the accumulation of organic material. Tangel has a high base saturation throughout the whole soil profile and shows a pH value above 5 in the transition zone to the carbonate bedrock or in the shallow mineral soil horizon.In this publication the preconditions for the development of Tangel are described, different forms of Tangel are characterized and a differentiation from other humus forms is made. On the example of the subalpine zone in the Wetterstein Mountains the coevolution of vegetation and Tangel humus forms as well as their degradation stages are discussed.

Surface-enhanced infrared absorption with Si-doped InAsSb/GaSb nano-antennas.

We demonstrate surface enhanced infrared absorption spectroscopy using 1-dimensional highly doped semiconductors based on Si-doped InAsSb plasmonic nano-antennas. Engineering the plasmonic array to support the localized surface plasmon resonance aligned with the molecular vibrational absorption mode of interest involves finely setting the doping level and nano-antenna width. Heavily doped nano-antennas require a wider size compared to lightly doped resonators. Increasing the doping level, and consequently the width of the nano-antenna, enhances the vibrational absorption of a ~15 nm thick organic layer up to 2 orders of magnitude compared to the unstructured sample and therefore improves sensing. These results pave the way towards molecule fingerprint sensor manufacturing by tailoring the plasmonic resonators to get a maximum surface enhanced infrared absorption at the target vibrational mode.

Predicting soil properties in the Canadian boreal forest with limited data: Comparison of spatial and non-spatial statistical approaches

Digital soil mapping (DSM) involves the use of georeferenced information and statistical models to map predictions and uncertainties related to soil properties. Many remote regions of the globe, such as boreal forest ecosystems, are characterized by low sampling efforts and limited availability of field soil data. Although DSM is an expanding topic in soil science, little guidance currently exists to select the appropriate combination of statistical methods and model formulation in the context of limited data availability. Using the Canadian managed forest as a case study, the main objective of this study was to investigate to which extent the choice of statistical method and model specification could improve the spatial prediction of soil properties with limited data. More specifically, we compared the cross-product performance of eight statistical approaches (linear, additive and geostatistical models, and four machine-learning techniques) and three model formulations (“covariates only”: a suite of environmental covariates only; “spatial only”: a function of geographic coordinates only; and “covariates+spatial”: a combination of both covariates and spatial functions) to predict five key forest soil properties in the organic layer (thickness and C:N ratio) and in the top 15cm of the mineral horizon (carbon concentration, percentage of sand, and bulk density). Our results show that 1) although strong differences in predictive performance occurred across all statistical approaches and model formulations, spatially explicit models consistently had higher R2 and lower RMSE values than non-spatial models for all soil properties, except for the C:N ratio; 2) Bayesian geostatistical models were among the best methods, followed by ordinary kriging and machine-learning methods; and 3) comparative analyses made it possible to identify the more performant models and statistical methods to predict specific soil properties. We make modeling tools and code available (e.g., Bayesian geostastical models) that increase DSM capabilities and support existing efforts toward the production of improved digital soil products with limited data.

Leaf and root litter decomposition is discontinued at high altitude tropical montane rainforests contributing to carbon sequestration.

We investigated how altitude affects the decomposition of leaf and root litter in the Andean tropical montane rainforest of southern Ecuador, that is, through changes in the litter quality between altitudes or other site‐specific differences in microenvironmental conditions. Leaf litter from three abundant tree species and roots of different diameter from sites at 1,000, 2,000, and 3,000 m were placed in litterbags and incubated for 6, 12, 24, 36, and 48 months. Environmental conditions at the three altitudes and the sampling time were the main factors driving litter decomposition, while origin, and therefore quality of the litter, was of minor importance. At 2,000 and 3,000 m decomposition of litter declined for 12 months reaching a limit value of ~50% of initial and not decomposing further for about 24 months. After 36 months, decomposition commenced at low rates resulting in an average of 37.9% and 44.4% of initial remaining after 48 months. In contrast, at 1,000 m decomposition continued for 48 months until only 10.9% of the initial litter mass remained. Changes in decomposition rates were paralleled by changes in microorganisms with microbial biomass decreasing after 24 months at 2,000 and 3,000 m, while varying little at 1,000 m. The results show that, irrespective of litter origin (1,000, 2,000, 3,000 m) and type (leaves, roots), unfavorable microenvironmental conditions at high altitudes inhibit decomposition processes resulting in the sequestration of carbon in thick organic layers.

Seasonal and Long-Term Changes to Active-Layer Temperatures after Tall Shrubland Expansion and Succession in Arctic Tundra

Vegetation, active-layer soils, and snow cover regulate energy exchange between the atmosphere and permafrost. Therefore, interactions between changes to tundra vegetation and soil thermal regime will fundamentally affect permafrost in a warmer world. We recorded soil temperatures for approximately 1 year in a Siberian Low Arctic landscape with a known history of alder (Alnus) shrub expansion on disturbed microsites in patterned ground. We recorded near-surface soil temperatures and measured physical properties of soils and vegetation on sorted-circle microsites in four stages of shrubland development: (1) tundra lacking tall shrubs; (2) shrub colonization zones; (3) mature shrublands; and (4) paludified, long-established shrublands with thick soil organic layers. Summer soil temperatures declined with increasing shrub cover and soil organic thickness; shrub colonization suppressed cryoturbation, facilitating the development of continuous vegetation and a surface organic mat on circles. Compared to open tundra, mature shrubs cooled soils by up to 9 °C during summer, but warmed soils by greater than 10 °C in winter presumably because they developed highly insulative snowpacks. Paludified shrublands had the coldest summer active layers, but winter soil temperatures were much colder than mature shrublands and were similar to earlier stages. Our results indicate that although tall shrub establishment dramatically warms winter soils within decades, much of this warming is transient at paludification-prone sites because the buildup of wet peat favors cooling in winter and the stature and snow-trapping capacity of shrubs diminish over time. In the ecosystem we studied, shrub expansion has contrasting effects on active-layer temperatures both seasonally and over longer timescales due to successional processes.

Characterizing hydrologic changes of the Great Dismal Swamp using SAR/InSAR

The Great Dismal Swamp (GDS), one of the largest, northernmost peatlands on the Atlantic Coastal Plain, is underlain by a thick water-logged organic soil layer (peat) made up of dead and decaying plant material. The peatland functions as a main sink for a large amount of soil derived organic carbon. The disturbance of this wetland has negatively impacted the ecosystem and contributed to climate change through the release of the stored greenhouse gases. Surface water level and soil moisture conditions are critical information about peatlands, but monitoring these hydrologic changes has been a challenging task. With a lack of in situ soil moisture measurements, we first explored yearlong Soil Moisture Active Passive (SMAP) data to find the close relationship (R-squared value: 0.80) between soil moisture and groundwater table from March 2015 to March 2016. Based on synthetic aperture radar (SAR) backscattering returns and interferometric SAR (InSAR) phase measurements from C-band Radarsat-1 and L-band ALOS PALSAR datasets, we then analyzed the hydrologic changes in the peatlands. We compared averaged C/L-band SAR backscattering intensity (mid 1998–early 2008 for Radarsat-1, late 2006–early 2011 for ALOS PALSAR) with groundwater level changes and found that the SAR backscattering is significantly responsive (R-squared value: 0.76 and 0.67 for Radarsat-1 and ALOS PALSAR, respectively) to soil moisture changes through a three-way correlated relationship of soil moisture, groundwater level, and SAR intensity. Using InSAR coherence observations, we delineated the inundated area (western and northern regions of GDS) during the wet season, subject to double-bounce backscattering. We measured the relative water level changes in the inundated areas through the InSAR phase measurements, and estimated the groundwater level changes corresponding to soil moisture changes using time-series InSAR analysis. Our comprehensive study has demonstrated that time-series SAR backscattering returns and InSAR analysis can be used to gauge soil moisture conditions and to monitor the hydrologic and vegetation changes in the GDS.

Advanced RF CMOS power amplifier with a monolithic high‐Q transformer on a thick laminated organic layer

AbstractWe propose a new transformer structure for output matching networks to improve the power‐added efficiency (PAE) of CMOS power amplifiers (PAs). To minimize the effects of a lossy silicon substrate, the transformer is realized on a very thick organic layer which is formed by lamination process. The maximum available gain of the transformer is improved from −1.23 to −0.68 dB as the insulator thickness is changed from 3 to 60 μm. As a result, the PAE of the proposed CMOS PA with the proposed monolithic high‐Q transformer shows a maximum output power of 24.98 dBm with a 30.49% PAE at 2.4 GHz.

Effect of the Organic Layer Thickness on the Device Performance of Organic/Inorganic Solar Cells Having 30% Cobalt’s Chloride Doped POT

Effect of the Organic Layer Thickness on the Device Performance of Organic/Inorganic Solar Cells Having 30% Cobalt’s Chloride Doped POT

Predicting Ecosystem Resilience to Fire from Tree Ring Analysis in Black Spruce Forests

Climate change has increased the occurrence, severity, and impact of disturbances on forested ecosystems worldwide, resulting in a need to identify factors that contribute to an ecosystem’s resilience or capacity to recover from disturbance. Forest resilience to disturbance may decline with climate change if mature trees are able to persist under stressful environmental conditions that do not permit successful recruitment and survival after a disturbance. In this study, we used the change in proportional representation of black spruce pre- to post-fire as a surrogate for resilience. We explored links between patterns of resilience and tree ring signals of drought stress across topographic moisture gradients within the boreal forest. We sampled 72 recently (2004) burned stands of black spruce in interior Alaska (USA); the relative dominance of black spruce after fire ranged from almost no change (high resilience) to a 90% decrease (low resilience). Variance partitioning analysis indicated that resilience was related to site environmental characteristics and climate–growth responses, with no unique contribution of pre-fire stand composition. The largest shifts in post-fire species composition occurred in sites that experienced the compounding effects of pre-fire drought stress and shallow post-fire organic layer thickness. These sites were generally located at warmer and drier landscape positions, suggesting they are less resilient to disturbance than sites in cool and moist locations. Climate–growth responses can provide an estimate of stand environmental stress to climate change and as such are a valuable tool for predicting landscape variations in forest ecosystem resilience.

Humus forms in relation to altitude and forest type in the Northern mountainous regions of Iran

This study focuses on the morphological and chemical development of humus forms as a function of altitude and forest type in the Northern Iran. Terrestrial humus forms related to European Humus Group proposal (EHGP) were evaluated under six common forest types, i.e. Alnus subcordata (AS), Acer velutinum (AV), Fagus orientalis - Carpinus betulus (FC), Carpinus betulus (CB), Fagus orientalis (FO) and Fagus orientalis - Carpinus betulus - Parrotia persica (FCP) stands with different topographic conditions. Within 61 permanent plots, organic(OL, OF, OH), organic-mineral (Ah) and mineral (soil A 0–15; first 15cm under Ah) layers were collected for identification and chemical analyses. Our results support the idea that altitude and species composition were the two major factors controlling the presence of the different humus forms and their chemical characteristics. While mull was mainly observed at the lowest altitude under AS, moder was usually present at middle altitude under FC and FCP and mor dominated at the highest altitude under FO and FC. Moreover, organic layer thickness also increased with altitude and was higher in the case of mor. On the contrary, the organic-mineral layer thickness was higher at the lowest elevations in the case of mull. The chemical composition of humus layers also differed among forest types. PCA analysis showed that AS and AV forest types were closely related with mull and amphi humus forms, pH, macro nutrients and soil C and N, while FO, FC, and FCP forest types were closely related with moder and mor humus forms, C:N ratio, slope and altitude.

Synthesis and photophysical properties of pyrene-functionalized nano-SiO2 hybrids in solutions and doped-PMMA thin films

Luminescent pyrene-functionalized nano-SiO2 (nano-SiO2Pyr) hybrids were synthesized and characterized using thermogravimetry, infrared, UV–vis absorption and, X-ray photoelectron spectroscopy, as well as field emission transmission electron microscopy (FETEM). The organic substituents immobilized on the nano-SiO2Pyr hybrids accounted for approximately 10% of the total weight. Polyethylene glycol 200 (PEG200) was found to be the most suitable solvent to suspend the nano-SiO2Pyr hybrids compared to other commonly used organic solvents. FETEM images indicated an average SiO2 nanoparticle diameter of approximately 12 nm and a 1- to 2-nm thick organic species functionalization layer. Several emission peaks were recorded at wavelengths of 380–580 nm and were designated as emissions arising from either the monomer or excimer of the pyrene substituents. Excimer formation was concentration and solvent polarity dependent, with higher concentrations and a stronger solvent polarity benefiting excimer formation. Further, nano-SiO2Pyr hybrids were doped in poly(methyl methacrylate) (PMMA) thin films; fluorescence spectra indicated that the excimer could be formed almost exclusively from neighboring nano-SiO2Pyr hybrids. Time-resolved fluorescence decays revealed that the emission lifetimes of nano-SiO2Pyr monomers and excimers were approximately 190 ns and 65–100 ns in the PEG200 solution, respectively, which was shortened to 0.45 ns to tens of ns in doped PMMA thin films, depending on the nano-hybrid concentration. Thus, the present study not only provides a method to prepare luminescent nano-materials but also a route to investigate excimer formation in solutions and thin films.

Structural composition of organic matter in particle-size fractions of soils along a climo-biosequence in the main range of Peninsular Malaysia

AbstractInformation on structural composition of organic matter (OM) in particle-size fractions of soils along a climo-biosequence is sparse. The objective of this study was to examine structural composition and morphological characteristics of OM in particle-size fractions of soils along a climo-biosequence in order to better understand the factors and processes affecting structural composition of soil organic matter. To explore changes in structural composition of OM in soils with different pedogenesis, the A-horizon was considered for further analyses including particle-size fractionation, solid-state13C nuclear magnetic resonance (NMR) spectroscopy and scanning electron microscopy (SEM). Due to the increase in the thickness of organic layer with increasing elevation, the A-horizon was situated at greater depth in soils of higher elevation. The relationship between relative abundances of carbon (C) structures and particle-size fractions was examined using principal component analysis (PCA). It was found that alkyl C (20.1-73.4%) and O-alkyl C (16.8-67.7%) dominated particle-size fractions. The proportion of alkyl C increased with increasing elevation, while O-alkyl C showed an opposite trend. Results of PCA confirmed this finding and showed the relative enrichment of alkyl C in soils of higher elevation. Increase in the proportion of alkyl C in 250-2000 μm fraction is linked to selective preservation of aliphatic compounds derived from root litter. SEM results showed an increase in root contribution to the 250-2000 μm fraction with increasing elevation. For the <53 μm fraction, pedogenic process of podzolization is responsible for the relative enrichment of alkyl C. This study demonstrates that changes in structural composition of OM in particle-size fractions of soils along the studied climo-biosequence are attributed to site-specific differences in pedogenesis as a function of climate and vegetation.

Landscape Effects of Wildfire on Permafrost Distribution in Interior Alaska Derived from Remote Sensing

Climate change coupled with an intensifying wildfire regime is becoming an important driver of permafrost loss and ecosystem change in the northern boreal forest. There is a growing need to understand the effects of fire on the spatial distribution of permafrost and its associated ecological consequences. We focus on the effects of fire a decade after disturbance in a rocky upland landscape in the interior Alaskan boreal forest. Our main objectives were to (1) map near-surface permafrost distribution and drainage classes and (2) analyze the controls over landscape-scale patterns of post-fire permafrost degradation. Relationships among remote sensing variables and field-based data on soil properties (temperature, moisture, organic layer thickness) and vegetation (plant community composition) were analyzed using correlation, regression, and ordination analyses. The remote sensing data we considered included spectral indices from optical datasets (Landsat 7 Enhanced Thematic Mapper Plus (ETM+) and Landsat 8 Operational Land Imager (OLI)), the principal components of a time series of radar backscatter (Advanced Land Observing Satellite—Phased Array type L-band Synthetic Aperture Radar (ALOS-PALSAR)), and topographic variables from a Light Detection and Ranging (LiDAR)-derived digital elevation model (DEM). We found strong empirical relationships between the normalized difference infrared index (NDII) and post-fire vegetation, soil moisture, and soil temperature, enabling us to indirectly map permafrost status and drainage class using regression-based models. The thickness of the insulating surface organic layer after fire, a measure of burn severity, was an important control over the extent of permafrost degradation. According to our classifications, 90% of the area considered to have experienced high severity burn (using the difference normalized burn ratio (dNBR)) lacked permafrost after fire. Permafrost thaw, in turn, likely increased drainage and resulted in drier surface soils. Burn severity also influenced plant community composition, which was tightly linked to soil temperature and moisture. Overall, interactions between burn severity, topography, and vegetation appear to control the distribution of near-surface permafrost and associated drainage conditions after disturbance.

Climate, soil organic layer, and nitrogen jointly drive forest development after fire in the North American boreal zone

AbstractPrevious empirical work has shown that feedbacks between fire severity, soil organic layer thickness, tree recruitment, and forest growth are important factors controlling carbon accumulation after fire disturbance. However, current boreal forest models inadequately simulate this feedback. We address this deficiency by updating the ED2 model to include a dynamic feedback between soil organic layer thickness, tree recruitment, and forest growth. The model is validated against observations spanning monthly to centennial time scales and ranging from Alaska to Quebec. We then quantify differences in forest development after fire disturbance resulting from changes in soil organic layer accumulation, temperature, nitrogen availability, and atmospheric CO2. First, we find that ED2 accurately reproduces observations when a dynamic soil organic layer is included. Second, simulations indicate that the presence of a thick soil organic layer after a mild fire disturbance decreases decomposition and productivity. The combination of the biological and physical effects increases or decreases total ecosystem carbon depending on local conditions. Third, with a 4°C temperature increase, some forests transition from undergoing succession to needleleaf forests to recruiting multiple cohorts of broadleaf trees, decreasing total ecosystem carbon by ∼40% after 300 years. However, the presence of a thick soil organic layer due to a persistently mild fire regime can prevent this transition and mediate carbon losses even under warmer temperatures. Fourth, nitrogen availability regulates successional dynamics; broadleaf species are less competitive with needleleaf trees under low nitrogen regimes. Fifth, the boreal forest shows additional short‐term capacity for carbon sequestration as atmospheric CO2 increases.

Controllable thin-film morphology and structure for 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8BTBT) based organic field-effect transistors

Organic field-effect transistors (OFETs) based on organic semiconductor material 2,7-dioctyl[1]benzothieno[3,2-b] benzothiophene (C8BTBT) as the active layer were fabricated by using organic molecular beam deposition (OMBD) and solution-processed methods, in which the C8BTBT thin-film morphology could be well controlled. In OMBD method, C8BTBT thin-film morphology could be controlled by the thickness of organic semiconductor layer and the deposition rate, of which the high-quality C8BTBT thin film was obtained at a thickness of about 20 nm and at a deposition rate of 1.2 nm/min, resulting in an obvious mobility improvement from 2.8 × 10−3 cm2 V−1 s−1 to 1.20 cm2 V−1 s−1. While in the solution-processing, C8BTBT thin-film morphology and thickness are related to the spin-coating speed and the substrate position in spin coater, i.e., in-centre and off-centre position. The off-centre spin-coating with an optimized speed produced large-size domain C8BTBT thin film and accordingly resulted in a mobility of 1.47 cm2 V−1 s−1. Furthermore, an additive polystyrene (PS) was added into C8BTBT solution could further improve the thin-film morphology with more metal-stable phase as well as improve the interface contact with the substrate SiO2, resulting in the highest mobility up to 3.56 cm2 V−1 s−1. The research suggested that C8BTBT-based OFETs with the mobility over 1.20 cm2 V−1 s−1 could be fabricated by using both OMBD and solution-processed methods through the thin-film morphology and structure optimization, which shows the potential applications in high-performance flexible and printed electronics.

Silviculture to sustain productivity in black spruce paludified forests

Fire is considered the major disturbance in boreal forests. Nonetheless, in several areas logging has become the primary driver of forest dynamics. In many areas of the boreal forest, stands may undergo paludification (i.e. the accumulation of thick, poorly decomposed organic layers over the mineral soil) in the prolonged absence of fire, which reduces forest productivity. Whereas high-severity fires (HSF) may restore forest productivity by burning the soil organic layer (SOL), low-severity fires (LSF) mainly burn the soil surface and do not significantly reduce SOL thickness. In the Clay Belt region of eastern Canada, an area prone to paludification, forest stands have historically been harvested by clearcutting (CC), but concerns about the protection of soils and tree regeneration lead to the replacement of CC by careful logging (CL). Whereas CC disturbs the SOL and is thought to favor tree growth, CL has little impact on the SOL. Furthermore, it has been suggested that prescribed burning after clearcut (CCPB) could also be used to control paludification. Using a retrospective approach, this study sought to understand how CC, CL, and CCPB compare to LSF and HSF with respect to soil properties, SOL thickness, vegetation ground cover, tree nutrition, and stand height in paludified black spruce stands of the Clay Belt region. HSF led to significantly taller trees than CL and LSF, but did not differ from CC and CCPB. Foliar N was significantly higher in HSF and CCPB sites relative to CL and LSF, with an intermediate value in CC sites. Ground cover of Rhododendron groenlandicum was significantly lower in HSF and CC sites relative to LSF, with intermediate values in CL and CCPB sites. Sphagnum spp. ground cover was significantly lower in HSF and CCPB sites relative to CL, with intermediate values in CC and LSF sites. High-severity fire sites had a significantly thinner SOL than the four other disturbances. Finally, regression tree analysis showed that SOL thickness represented the best predictor of tree height, whereas segmented regression showed that tree height was negatively correlated to SOL thickness and revealed a cut-off point circa 23cm, which suggests that tree growth is impeded beyond this threshold. These results support the idea that management strategies intending to regenerate paludified forests should primarily aim at reducing organic layer thickness, either through mechanical disturbance or combustion.

Influences and interactions of inundation, peat, and snow on active layer thickness

AbstractActive layer thickness (ALT), the uppermost layer of soil that thaws on an annual basis, is a direct control on the amount of organic carbon potentially available for decomposition and release to the atmosphere as carbon‐rich Arctic permafrost soils thaw in a warming climate. We investigate how key site characteristics affect ALT using an integrated surface/subsurface permafrost thermal hydrology model. ALT is most sensitive to organic layer thickness followed by snow depth but is relatively insensitive to the amount of water on the landscape with other conditions held fixed. The weak ALT sensitivity to subsurface saturation suggests that changes in Arctic landscape hydrology may only have a minor effect on future ALT. However, surface inundation amplifies the sensitivities to the other parameters and under large snowpacks can trigger the formation of near‐surface taliks.

Ballistic electron and photocurrent transport in Au/organic/Si(001) diodes with PDI8-CN2 interlayers

The authors use ballistic electron emission microscopy (BEEM) to probe hot-electron and photocurrent transport in Au/organic/n-Si(001) diodes incorporating the n-type perylene diimide semiconductor PDI8-CN2. For the case of an ultrathin organic interlayer, hot-electron injection is weak and can be detected only at randomly distributed nanosized domains, where BEEM provides electronic barrier heights of ∼0.67 and ∼0.94 eV, respectively. No ballistic transport is detected for devices with a 10 nm-thick interlayer. Regardless of the organic layer thickness, BEEM reveals laterally uniform contributions due to scanning tunneling microscopy-induced photocurrent (STM-PC), with a characteristic energy onset at ∼1.2 eV and a broad intensity peak in the 2–4 eV range. The authors give insight on such spectroscopic features by examination of temperature-dependent spectra and of literature data. This study shows that PDI8-CN2 limits the penetration of Au toward Si, likely due to stiff intermolecular interactions and reactivity of the cyano groups. Moreover, ballistic transmittance is remarkably suppressed and photocurrent transport takes place via defects or recombination centers. Our analysis of electronic and STM-PC fingerprints appears useful for the characterization of several organic-on-inorganic interfaces of interest for heterostructures and devices.